KotOR 2 -Improved pretty much every important gameplay aspect over KotOR. It's a shame Lucasarts pushed them to a release for a holiday deadline
Fallout New Vegas -Improved pretty much every complaint against Fallout 3. The only negative was the exploration and open-ness weren't quite where I wanted them.
Battlefront 2 -Added new units and the ability to wreak havoc as a Jedi, along with improved game modes such as Galactic Conquest.
Pokemon Gold/Silver -Took everything everyone loved about Red and Blue and doubled it.
Fable 2 -Improved the scale of the game, added new enemies, new weapon types, more things to do, just more of everything. Although the first game was great, this one was better, imo.
Jed Knight: Dark Forces II -This was the first Star Wars game where you got to use a light saber in full 3D from a first-person perspective, and it felt awesome. Also the level design was great and, every single map felt gigantic.
Spiderman 2 -The definitive superhero game until the Arkham games showed up. It had an open world, with tons of buildings to climb and things to collect and varied missions to do and it felt like you were Spiderman. --- Would you kindly do a barrel roll? War. War never changes.
assuming that modern warfare is its own series, i personally think that mw2 is better than cod 4. the campaign is worse but the multiplayer was heavily expanded and whilst cod 4 is still the base model from which every cod game since has followed, mw2 dominated xbox live and psn for the next year basically. also staying with cod, black ops 2 is undeniably better that the original black ops. portal 2 was also better, and assassins creed 2 was an extreme improvement over the first.