A couple of Pokemon i used on my playthrough. Since I have only have one copy of Plat though, and all my "good" Pokemon are in there, I'm stuck using Pearl for ingame :(
Skorupi - C rank - It levels up slow, evolves into Drapion at level 40, and Pin Missle is your strongest level up move until level 34, when it gets Bug Bite. That makes Skorupi pretty bad. Drapion is fairly good, however, if you get it. Crunch, Cross Poison, and the Elemental Fangs are nice, and it can learn Earthquake if you want something stronger than Fire Fang for Steels. Dark/Poison is a fairly good typing, and Bryon is the only Gym that should cause it trouble. Also, Sniper is pretty cool with Cross Poison. Levels up too slow and evolves too late to be better, though.
Tropius - C Rank - Good defenses, and access to Eathquake, Air Slash, Synthesis and Leaf Storm is nice. The ability to learn 5 HMs makes it a great HM slave, too. However, it's Attack, Speed, and Special Attack are bit too low to be B rank imo.
Scyther - S rank - It is available right before Maylene, at around level 20, which is a little low, but not too bad. If you get Technician, Wing Attack is very strong. False Swipe makes it great for catching other Pokemon, and it natually learns Swords Dance, Night Slash and X-scissor. Unless you trade it, it will have trouble with the last three gyms though. If you do evolve it into Scizor, use a Heart Scale to teach it Bullet Punch and you have a monster. It can single handily take down Candice's Gym, and most of team Galatic has trouble with it thanks to it's Steel-typing (Houndour and Flamethrower Skuntank are trouble though). X-scissor also makes it great against Lucian. Just avoid Fire moves like the plague, and Scizor should do fine.
Magby/Magmar - A rank - The best fire-type after Chimchar and maybe Houdour imo. It doesn't even need to evolve into Magmortar to be good. You need to get to Surf to the Fuego Ironworks to catch Magmar. It can be caught at level 29, which is fairly low compared to the rest of your Pokemon. However, if you teach it Flamethrower through TM or Lava Plume at level 36, it will destroy Bryon's and Candice's gyms. It's learns mostly Fire moves from leveling up, which is bad. It can learn Thunderpunch from the tutor though. If you evolve it into Magmortar, it gains the ability to learn T-bolt. Magmar is fairly fast (faster than it's evo, which is base 83) and has strong attacking stats. I would say it's S rank, but it's level up movepool is too shallow and you get it a bit late for it to be better, though. A great ingame Pokemon overall.
Carnivine - D rank - It's not terrible, but Tangela, Turtwig, Budew and maybe Tropius are so much better. It's level up movepool is pretty barren, since it only learns Dark- and Grass-type attacking moves. Stockpile is neat, but not useful on something like Carnivine. If you breed it, you can get Sleep Powder, but Tangela get's it from a Heart Scale, so why bother? Its Attack and Sp.Att are nice, but it's slow as hell and has few special moves to use. It is pretty much the worst grass-type in the game. It does look cool, however, and that must count for something ingame, right?
edit: for some reason, i only saw the posts on page 1. Now i feel like i'm saying what has already been said :(