After talking for a bit on IRC, several (well, 2 or 3) people decided that we should have an in-game tier thread for DPPt. There isn't one being updated (the last one is (was?) in Stark Mountain), so here we are.
Now, it's been a long time since I played Platinum, so I can't really give anything to this article, but you can help! Just toss a reply right here. Theorymoning, while not prohibited, will probably not get your article in the OP.
Version : All
Stats: Starly and its evoultions have great Attack and Speed, but middling defenses and horrid Special Attack. Intimidate somewhat fixes your bulk issues. Fully evolved, Staraptors base 120 Attack and 100 Speed leave it far ahead of the pack. The good news here is, you evolve into Staravia at level 14, and then evolve into Staraptor at level 34, which is a fairly quick evolution line.
Movepool: Starly's evolution line has a fantastic movepool, boasting 2 fearsome STAB moves in Brave Bird and Return (Double-Edge and Facade work well here too), and you then get Close Combat at level 34 as a Staraptor when you evolve, which finishes off your coverage by hitting the Steel and Rock types currently walling your STAB's. Your 4th moveslot can even be put to use to learn Fly, an in demand move in-game.
Availability: Very early, before you even reach your first gym you will have had multiple chances to catch Starly on the first 2 routes you use in the game at a very high apprentice rate, and if you miss out there Staravia can be caught during the middle parts of the game as well.
Version: Plat.
If you didn't pick Chimchar, you have very few choices in terms of fire types. This is the 3rd best fire type behind Infernape and Houndoom. It can sweep gyms and Elite Four members by itself, but it evolves at the high level of 40.
Availability: Early, Right outside Oreburgh City.
Stats: Rapidash has got a great 105 speed, and a decent 100 base attack, and a slightly disappointing Sp. Atk with 80.
Moveset: Moveset wise, it doesn't have much going for it. It can use various Fire STABs, such as Fire Blast and Flamethrower, but that can get you by. It's best physical STAB is Flare Blitz, which sadly you don't get until 56. It also gets Megahorn through Heart Scale, which can be an option against Lucian.
Version: All
Stats: Poor. Pachirisu has terrible HP, Attack, Defense, and Special Attack compared to other available Pokémon. At its time of first encounter, though, it is one of the fastest Pokémon, and also has decent Special Defense. Its stats lose luster as soon as others begin evolving, however, as when you encounter it your starter should have evolved, at least.
Movepool: Shallow. There are only a few moves worth using on Pachirisu, and they are as follows: Quick Attack, Super Fang, and Discharge. If you use it for anything else, it will be outclassed even when you first meet it, seeing as Shinx/Luxio has both higher Attack and learns Spark around the same time. All of its other moves can also be matched by others around the time of encounter. Also, Super Fang comes mid-game, at Level 33, when Pachirisu is no longer a valuable party member. It does learn Cut and Flash, though.
Availability: Early. Pachirisu is available just before Eterna Forest at the Valley Windworks. He can be of some assistance against Team Galactic, and can even be grinded with Cheryl in the Eterna Forest, which can be used to level up easily against the resident Budew and Trainers' Abra and Beautifly. It also comes around the time when an HM slave for Cut can be used, so that be harped upon.
Matchups
Additional Comments: None, really. Try to avoid using Pachirisu. It's cute and all, but not the greatest Pokémon for an efficient run. It really it cute, though, so you could keep it as a mascot, like Ash's Pikachu.
Form to fill out:
Game version(s): Plat., DP, or both.
Stats: This should explain itself.
Movepool: Again, this should be obvious.
Availability: Include version exclusiveness.
Additional comments: Boss matchups (not too in depth, just the really important bits), other tidbits that would be useful.
If this form is not filled out correctly, your entry WILL be ignored.
We're assuming that people can obtain trade evos, but some Pokemon that are useful on their own without a trade evo can be considered (will always be placed at least one tier lower).
Now, it's been a long time since I played Platinum, so I can't really give anything to this article, but you can help! Just toss a reply right here. Theorymoning, while not prohibited, will probably not get your article in the OP.
Abra
Azelf
Bidoof reserved for Naix
Buizel
Chimchar reserved for Naix
Cranidos reserved for Naix
Gastly
Gastly
Gible
Giratina
Houndour
Machop
Piplup (reserved for Espeon65)
Rotom
Scyther
Shink
Starly Done.
Turtwig
Bronzor
Croagunk
Snorunt-->Froslass
Ralts-->Gallade reserved for Naix, not sure which evo
Ralts-->Gardevoir reserved for Naix, not sure which evo
Eevee-->Jolteon
Magikarp
Marill
Meditite
Pikachu
Ponyta Done.
Shellos
Eevee-->Vaporeon
Bonsly
Budew reserved for Naix
Buneary
Chatot
Clefairy
Drifloon Done.
Electabuzz
Geodude
Girafarig
Eevee-->Glaceon
Gligar
Hoothoot
Magnemite
Mesprit
Psyduck
Uxie
Wingull
Wooper
Zubat
Absol
Aipom
Barboach
Burmy
Carnivine
Cherubi
Chansey
Chingling
Combee
Duskull
Eevee-->Espeon (reserved for Espeon65)
Feeba
Finneon
Eevee-->Flareon
Snorunt-->Glalie
Goldeen
Heracross
Hippopotas
Kricketot
Eevee-->Leafeon
Lickitung
Magmar
Mantyke
Mime Jr.
Munchlax
Nosepass
Onix
Pachirisu Done.
Porygon
Remoraid
Rhyhorn
Riolu
Shieldon
Skorupi
Sneasel
Snover
Spritomb
Swablu
Swinub
Tangela
Tentacool
Togepi
Tropius
Eevee-->Umbreon
Unown
Wurmple
Yanma
Dialga
Palkia
Azelf
Bidoof reserved for Naix
Buizel
Chimchar reserved for Naix
Cranidos reserved for Naix
Gastly
Gastly
Gible
Giratina
Houndour
Machop
Piplup (reserved for Espeon65)
Rotom
Scyther
Shink
Starly Done.
Turtwig
Bronzor
Croagunk
Snorunt-->Froslass
Ralts-->Gallade reserved for Naix, not sure which evo
Ralts-->Gardevoir reserved for Naix, not sure which evo
Eevee-->Jolteon
Magikarp
Marill
Meditite
Pikachu
Ponyta Done.
Shellos
Eevee-->Vaporeon
Bonsly
Budew reserved for Naix
Buneary
Chatot
Clefairy
Drifloon Done.
Electabuzz
Geodude
Girafarig
Eevee-->Glaceon
Gligar
Hoothoot
Magnemite
Mesprit
Psyduck
Uxie
Wingull
Wooper
Zubat
Absol
Aipom
Barboach
Burmy
Carnivine
Cherubi
Chansey
Chingling
Combee
Duskull
Eevee-->Espeon (reserved for Espeon65)
Feeba
Finneon
Eevee-->Flareon
Snorunt-->Glalie
Goldeen
Heracross
Hippopotas
Kricketot
Eevee-->Leafeon
Lickitung
Magmar
Mantyke
Mime Jr.
Munchlax
Nosepass
Onix
Pachirisu Done.
Porygon
Remoraid
Rhyhorn
Riolu
Shieldon
Skorupi
Sneasel
Snover
Spritomb
Swablu
Swinub
Tangela
Tentacool
Togepi
Tropius
Eevee-->Umbreon
Unown
Wurmple
Yanma
Dialga
Palkia

Version : All
Stats: Starly and its evoultions have great Attack and Speed, but middling defenses and horrid Special Attack. Intimidate somewhat fixes your bulk issues. Fully evolved, Staraptors base 120 Attack and 100 Speed leave it far ahead of the pack. The good news here is, you evolve into Staravia at level 14, and then evolve into Staraptor at level 34, which is a fairly quick evolution line.
Movepool: Starly's evolution line has a fantastic movepool, boasting 2 fearsome STAB moves in Brave Bird and Return (Double-Edge and Facade work well here too), and you then get Close Combat at level 34 as a Staraptor when you evolve, which finishes off your coverage by hitting the Steel and Rock types currently walling your STAB's. Your 4th moveslot can even be put to use to learn Fly, an in demand move in-game.
Availability: Very early, before you even reach your first gym you will have had multiple chances to catch Starly on the first 2 routes you use in the game at a very high apprentice rate, and if you miss out there Staravia can be caught during the middle parts of the game as well.
Coming soon to a thread near you!

Version: Plat.
If you didn't pick Chimchar, you have very few choices in terms of fire types. This is the 3rd best fire type behind Infernape and Houndoom. It can sweep gyms and Elite Four members by itself, but it evolves at the high level of 40.
Availability: Early, Right outside Oreburgh City.
Stats: Rapidash has got a great 105 speed, and a decent 100 base attack, and a slightly disappointing Sp. Atk with 80.
Moveset: Moveset wise, it doesn't have much going for it. It can use various Fire STABs, such as Fire Blast and Flamethrower, but that can get you by. It's best physical STAB is Flare Blitz, which sadly you don't get until 56. It also gets Megahorn through Heart Scale, which can be an option against Lucian.
Drifloon (Low Tier)
Version: All
Stats: Below Average. Besides HP and arguably Special Attack, Drifloon has some pretty below average stats. He's sort of fast, but his defenses are very, very lackluster to accompany his HP, and they really bring Drifloon down as a whole. His typing is decent, though. When he evolves into Drifblim, none of these facts really change. Other Pokémon really catch up quickly to him, and it gets harder and harder to win battles with his only ok offensive stats. He is also very easily outclassed by other Flying-types.
Movepool: Decent. Drifloon, when it evolves, can use a fairly wide range of attacks, from Shadow Ball to Payback, from Thunderbolt to Fly. He doesn't learn all that great STAB moves, though, having to rely on Astonish as a Ghost-type move until Ominous Wind, and even then Ominous Wind has terrible PP. The same goes for Flying-type STABs, seeing as Gust is the only Level-up move he learns.
Availability: Early. Drifloon is available on Fridays at the Valley Windworks after you beat Team Plasma, just before the Eterna Forest. Just reset your DS clock and you're golden. Eterna is a great place to level up, and the Eterna gym is designed specifically for Drifloon, it seems.
Additional Comments: Drifloon is cute and fun to use early on, but definitely takes a massive fall as soon as Maylene sends out that Lucario. It's all downhill from there. But he can function as an HM slave with Fly to an extent, right? Wrong. Unless you already have Drifblim, I would recommend leaving being a Fly slave to a Starly or something, as Drifloon cannot learn Fly until it evolves at Lv. 28. What a shame, though. He's just so cute.
Version: All
Stats: Below Average. Besides HP and arguably Special Attack, Drifloon has some pretty below average stats. He's sort of fast, but his defenses are very, very lackluster to accompany his HP, and they really bring Drifloon down as a whole. His typing is decent, though. When he evolves into Drifblim, none of these facts really change. Other Pokémon really catch up quickly to him, and it gets harder and harder to win battles with his only ok offensive stats. He is also very easily outclassed by other Flying-types.
Movepool: Decent. Drifloon, when it evolves, can use a fairly wide range of attacks, from Shadow Ball to Payback, from Thunderbolt to Fly. He doesn't learn all that great STAB moves, though, having to rely on Astonish as a Ghost-type move until Ominous Wind, and even then Ominous Wind has terrible PP. The same goes for Flying-type STABs, seeing as Gust is the only Level-up move he learns.
Availability: Early. Drifloon is available on Fridays at the Valley Windworks after you beat Team Plasma, just before the Eterna Forest. Just reset your DS clock and you're golden. Eterna is a great place to level up, and the Eterna gym is designed specifically for Drifloon, it seems.
Additional Comments: Drifloon is cute and fun to use early on, but definitely takes a massive fall as soon as Maylene sends out that Lucario. It's all downhill from there. But he can function as an HM slave with Fly to an extent, right? Wrong. Unless you already have Drifblim, I would recommend leaving being a Fly slave to a Starly or something, as Drifloon cannot learn Fly until it evolves at Lv. 28. What a shame, though. He's just so cute.

Version: All
Stats: Poor. Pachirisu has terrible HP, Attack, Defense, and Special Attack compared to other available Pokémon. At its time of first encounter, though, it is one of the fastest Pokémon, and also has decent Special Defense. Its stats lose luster as soon as others begin evolving, however, as when you encounter it your starter should have evolved, at least.
Movepool: Shallow. There are only a few moves worth using on Pachirisu, and they are as follows: Quick Attack, Super Fang, and Discharge. If you use it for anything else, it will be outclassed even when you first meet it, seeing as Shinx/Luxio has both higher Attack and learns Spark around the same time. All of its other moves can also be matched by others around the time of encounter. Also, Super Fang comes mid-game, at Level 33, when Pachirisu is no longer a valuable party member. It does learn Cut and Flash, though.
Availability: Early. Pachirisu is available just before Eterna Forest at the Valley Windworks. He can be of some assistance against Team Galactic, and can even be grinded with Cheryl in the Eterna Forest, which can be used to level up easily against the resident Budew and Trainers' Abra and Beautifly. It also comes around the time when an HM slave for Cut can be used, so that be harped upon.
Matchups
Additional Comments: None, really. Try to avoid using Pachirisu. It's cute and all, but not the greatest Pokémon for an efficient run. It really it cute, though, so you could keep it as a mascot, like Ash's Pikachu.
Form to fill out:
Game version(s): Plat., DP, or both.
Stats: This should explain itself.
Movepool: Again, this should be obvious.
Availability: Include version exclusiveness.
Additional comments: Boss matchups (not too in depth, just the really important bits), other tidbits that would be useful.
If this form is not filled out correctly, your entry WILL be ignored.
We're assuming that people can obtain trade evos, but some Pokemon that are useful on their own without a trade evo can be considered (will always be placed at least one tier lower).