Leaving out Pokemon like Magmortar and Togekiss because they were meant to be improved versions of other Pokemon, unless that Pokemon came after Generation 2.
Slaking
Type: Normal
BST: 150 / 160 / 100 / 95 / 65 / 100
Placement in the Metagames: BL / NU / NU
Tier in GSC: Uber
Slaking is a menace. Sporting 160/90 offenses, a good offensive movepool, access to Curse, and no Truant, it would maim anything in front of it. It can easily just maul anything in sight of it, and roast those Steels that dare attempt to tank its Body Slam with a beefy Fire Blast. It is no defensive slouch either, as it has 150/100/65 bulk that can tank a lot of hits, despite an iffy base Special Defense. There isn't much else to say about it.
Rampardos
Type: Rock
BST: 97/165/60/65/50/58
Placement in the Metagames: NU / NU
Tier in GSC: Uber
Example moveset: Curse / Rock Slide / Body Slam / Earthquake
Despite being very fragile and slow, Rampardos would have been broken in GSC OU It could 2HKO Skarmory at +1 with Rock Slide, and it can learn both Swords Dance and Curse, making it an unrivaled source of power. So much power would have undermined all of the stall in the metagame, and it would have just ran around punching holes through every team. Even in Ubers, it could do a lot of damage to Ho-Oh and Lugia, and would have increased the viability of Mewtwo by a lot.
Drifblim
Type: Ghost / Flying
BST: 150/80/44/90/54/80
Placement in the Metagames: NU / NU
Tier in GSC: Top-Tier OU
Example moveset: Rest / Sleep Talk / Shadow Ball / Thunderbolt
Despite having really shitty Defense and Special Defense stats to back up a colossal 150 base HP, Drifblim would have hard countered Snorlax, being immune to Body Slam and Earthquake, and being able to ward off Fire Blast, forcing Snorlax to use Rock Slide, which it really does not want to use, because then it would have coverage issues with Rock types or Steel types. It wouldn't mind running RestTalk to absorb a Lovely Kiss either, as Drifblim does not have any other method of recovery. Its 80/90 offenses and base 80 speed are not anything outstanding, but they are enough to make use of Shadow Ball, Body Slam, Thunderbolt, Thunder Wave, Thunder, and a potential Hidden Power. It also has access to Hypnosis and Destiny Bond, which is always great for taking down an opponent.
Banette
Type: Ghost
BST: 64/115/65/83/63/65
Placement in the Metagames: UU / NU / NU
Tier in GSC: Low OU
Example moveset: Shadow Ball / Thunder Wave / Thunderbolt / Destiny Bond
Despite being rather fragile, Bannette would have been a good offensive Ghost because unlike Gengar and Misdreavus, it has a solid base 115 Attack to back up a STAB Shadow Ball. Its movepool is rather small, but it does have access to Thunderbolt, Thunder Wave, and Body Slam, and Thunderbolt off an acceptable base 83 Special Attack is great for hitting Skarmory, who would have otherwise walled Bannette. Thunder Wave and Body Slam help alleviate its Speed issues, and it can always run Hidden Power Fighting to beat Normal types like Miltank and Snorlax. Bannette also has access to Destiny Bond to score a free KO against something that might KO it, like Vaporeon or Rhydon.
Regigigas
Type: Normal
BST: 110/160/110/80/110/100
Placement in the Metagames: NU / NU
Tier in GSC: Uber
Example Moveset: Substitute / Return / Earthquake / Thunder
Behold a metagame of no abilities, where Regigigas can wreak destruction. Despite having a very small movepool, it could rip the opponent apart with a colossal 160 base Attack. It could also just zap Skarmory with Thunder, preventing it from setting up Curses on it. It doesn't even need Curse and Body Slam, two moves it cannot learn, because it has so much Attack power to begin with. A definite fear for anyone that crosses paths with it.
Mesprit
Type: Psychic
BST: 80/105/105/105/105/80
Placement in the Metagames: UU / RU
Tier in GSC: Mid-Tier OU
Example Moveset: Thunder Wave / Psychic / Thunderbolt / Ice Beam
Mesprit is very well-balanced for the GSC metagame. Sporting 80/105/105 defenses, 105 base Special Attack, 80 base speed, it is a solid choice for a special attacker. Despite not having any physical attacks that can muscle past Blissey, Mesprit can put a lot of pressure on a lot of good Pokemon in the metagame with Psychic/Electric/Ice coverage. It has all of the qualities an offensive team in GSC would look for in a special attacker (bar Growth and Recover), and can thus be a very useful Pokemon.
Froslass
Type: Ice / Ghost
BST: 70/80/70/80/70/110
Placement in the Metagames: BL / BL
Tier in GSC: Low OU
Example Moveset: Spikes / Shadow Ball / Ice Beam / Thunder
Froslass looks out of place in a stall-based metagame with such low defenses and average offenses, but it actually can hit opponents harder than you think. For example, Nidoking's Special Attack is only 5 points higher than Froslass, and he is a good choice for a mixed attacker in OU. Also, Froslass can 3HKO Suicune with Thunder after Spikes, forcing it to scramble for Rest. But offenses are not why Froslass would be OU; the ability to spinblock and lay down Spikes at the same time as a great attribute, especially since Froslass has the potential beat every Rapid Spinner in OU. Although Froslass is rather fragile, keeping her limitations in mind can make her a decent threat in OU.
Hariyama
Type: Fighting
BST: 144/120/60/40/60/50
Placement in the Metagames: BL / UU / RU
Tier in GSC: Top-tier OU
Example moveset: Belly Drum / Cross Chop / Body Slam / Rock Slide
With all of the Snorlax running around, Machamp became viable for preventing Snorlax from switching in on it. Hariyama has one thing that Machamp does not have: Belly Drum. By reaching +6 attack in one turn, Hariyama can decimate paralyzed teams in seconds, destroying things with 999 attack. Despite having low PP and accuracy, STAB Cross Chop will hammer everything that does not resist it, making Hariyama even more threatening. To puts its power into perspective, Hariyama OHKOs Skarmory with a +6 Cross Chop. Despite having pitiful defenses backing up such a high HP rating, Hariyama would be a dangerous threat that every team needed a check to.
Torterra
Type: Grass / Ground
BST: 95/109/105/75/85/56
Placement in the Metagames: UU / NU
Tier in GSC: Mid-tier OU
Example moveset: Curse / Earthquake / Rock Slide / Body Slam
With key resistances to both Ground and Rock type moves, Torterra makes for a very decent physical tank. It also has great 95/105/85 bulk and a good base 109 attack. All of these attributes make for a good physical tank that can check threats like Tyranitar, Rhydon, and Steelix. It only has two physical weaknesses in Flying (which is rare) and Bug (which only Heracross, Forretress, and Marowak commonly carry), and thus can tank better than you think. Although its 4x weakness to Ice leaves it rather vulnerable from Special Attackers, Torterra is a decent threat in GSC OU.
Other Pokemon I have not examined yet:
Drapion, Cresselia, Exploud, Dusknoir, GO GO CLAMPERL AND SPINDA, Armaldo, Gastrodon
Slaking
Type: Normal
BST: 150 / 160 / 100 / 95 / 65 / 100
Placement in the Metagames: BL / NU / NU
Tier in GSC: Uber
Slaking is a menace. Sporting 160/90 offenses, a good offensive movepool, access to Curse, and no Truant, it would maim anything in front of it. It can easily just maul anything in sight of it, and roast those Steels that dare attempt to tank its Body Slam with a beefy Fire Blast. It is no defensive slouch either, as it has 150/100/65 bulk that can tank a lot of hits, despite an iffy base Special Defense. There isn't much else to say about it.
Rampardos
Type: Rock
BST: 97/165/60/65/50/58
Placement in the Metagames: NU / NU
Tier in GSC: Uber
Example moveset: Curse / Rock Slide / Body Slam / Earthquake
Despite being very fragile and slow, Rampardos would have been broken in GSC OU It could 2HKO Skarmory at +1 with Rock Slide, and it can learn both Swords Dance and Curse, making it an unrivaled source of power. So much power would have undermined all of the stall in the metagame, and it would have just ran around punching holes through every team. Even in Ubers, it could do a lot of damage to Ho-Oh and Lugia, and would have increased the viability of Mewtwo by a lot.
Drifblim
Type: Ghost / Flying
BST: 150/80/44/90/54/80
Placement in the Metagames: NU / NU
Tier in GSC: Top-Tier OU
Example moveset: Rest / Sleep Talk / Shadow Ball / Thunderbolt
Despite having really shitty Defense and Special Defense stats to back up a colossal 150 base HP, Drifblim would have hard countered Snorlax, being immune to Body Slam and Earthquake, and being able to ward off Fire Blast, forcing Snorlax to use Rock Slide, which it really does not want to use, because then it would have coverage issues with Rock types or Steel types. It wouldn't mind running RestTalk to absorb a Lovely Kiss either, as Drifblim does not have any other method of recovery. Its 80/90 offenses and base 80 speed are not anything outstanding, but they are enough to make use of Shadow Ball, Body Slam, Thunderbolt, Thunder Wave, Thunder, and a potential Hidden Power. It also has access to Hypnosis and Destiny Bond, which is always great for taking down an opponent.
Banette
Type: Ghost
BST: 64/115/65/83/63/65
Placement in the Metagames: UU / NU / NU
Tier in GSC: Low OU
Example moveset: Shadow Ball / Thunder Wave / Thunderbolt / Destiny Bond
Despite being rather fragile, Bannette would have been a good offensive Ghost because unlike Gengar and Misdreavus, it has a solid base 115 Attack to back up a STAB Shadow Ball. Its movepool is rather small, but it does have access to Thunderbolt, Thunder Wave, and Body Slam, and Thunderbolt off an acceptable base 83 Special Attack is great for hitting Skarmory, who would have otherwise walled Bannette. Thunder Wave and Body Slam help alleviate its Speed issues, and it can always run Hidden Power Fighting to beat Normal types like Miltank and Snorlax. Bannette also has access to Destiny Bond to score a free KO against something that might KO it, like Vaporeon or Rhydon.
Regigigas
Type: Normal
BST: 110/160/110/80/110/100
Placement in the Metagames: NU / NU
Tier in GSC: Uber
Example Moveset: Substitute / Return / Earthquake / Thunder
Behold a metagame of no abilities, where Regigigas can wreak destruction. Despite having a very small movepool, it could rip the opponent apart with a colossal 160 base Attack. It could also just zap Skarmory with Thunder, preventing it from setting up Curses on it. It doesn't even need Curse and Body Slam, two moves it cannot learn, because it has so much Attack power to begin with. A definite fear for anyone that crosses paths with it.
Mesprit
Type: Psychic
BST: 80/105/105/105/105/80
Placement in the Metagames: UU / RU
Tier in GSC: Mid-Tier OU
Example Moveset: Thunder Wave / Psychic / Thunderbolt / Ice Beam
Mesprit is very well-balanced for the GSC metagame. Sporting 80/105/105 defenses, 105 base Special Attack, 80 base speed, it is a solid choice for a special attacker. Despite not having any physical attacks that can muscle past Blissey, Mesprit can put a lot of pressure on a lot of good Pokemon in the metagame with Psychic/Electric/Ice coverage. It has all of the qualities an offensive team in GSC would look for in a special attacker (bar Growth and Recover), and can thus be a very useful Pokemon.
Froslass
Type: Ice / Ghost
BST: 70/80/70/80/70/110
Placement in the Metagames: BL / BL
Tier in GSC: Low OU
Example Moveset: Spikes / Shadow Ball / Ice Beam / Thunder
Froslass looks out of place in a stall-based metagame with such low defenses and average offenses, but it actually can hit opponents harder than you think. For example, Nidoking's Special Attack is only 5 points higher than Froslass, and he is a good choice for a mixed attacker in OU. Also, Froslass can 3HKO Suicune with Thunder after Spikes, forcing it to scramble for Rest. But offenses are not why Froslass would be OU; the ability to spinblock and lay down Spikes at the same time as a great attribute, especially since Froslass has the potential beat every Rapid Spinner in OU. Although Froslass is rather fragile, keeping her limitations in mind can make her a decent threat in OU.
Hariyama
Type: Fighting
BST: 144/120/60/40/60/50
Placement in the Metagames: BL / UU / RU
Tier in GSC: Top-tier OU
Example moveset: Belly Drum / Cross Chop / Body Slam / Rock Slide
With all of the Snorlax running around, Machamp became viable for preventing Snorlax from switching in on it. Hariyama has one thing that Machamp does not have: Belly Drum. By reaching +6 attack in one turn, Hariyama can decimate paralyzed teams in seconds, destroying things with 999 attack. Despite having low PP and accuracy, STAB Cross Chop will hammer everything that does not resist it, making Hariyama even more threatening. To puts its power into perspective, Hariyama OHKOs Skarmory with a +6 Cross Chop. Despite having pitiful defenses backing up such a high HP rating, Hariyama would be a dangerous threat that every team needed a check to.
Torterra
Type: Grass / Ground
BST: 95/109/105/75/85/56
Placement in the Metagames: UU / NU
Tier in GSC: Mid-tier OU
Example moveset: Curse / Earthquake / Rock Slide / Body Slam
With key resistances to both Ground and Rock type moves, Torterra makes for a very decent physical tank. It also has great 95/105/85 bulk and a good base 109 attack. All of these attributes make for a good physical tank that can check threats like Tyranitar, Rhydon, and Steelix. It only has two physical weaknesses in Flying (which is rare) and Bug (which only Heracross, Forretress, and Marowak commonly carry), and thus can tank better than you think. Although its 4x weakness to Ice leaves it rather vulnerable from Special Attackers, Torterra is a decent threat in GSC OU.
Other Pokemon I have not examined yet:
Drapion, Cresselia, Exploud, Dusknoir, GO GO CLAMPERL AND SPINDA, Armaldo, Gastrodon
