Assassin's Creed 2- I love the first AC game but the second one was one of the best games I have ever played. Great historical setting and characters along with some of the best mix of stealth and adventure gameplay.
Halo 2- This game had a lot of hype to live up to and for me it hit it right on the head. The story continued the grand epic and further established the universes as one of the best. More importantly though, Halo 2 brought competitive multiplayer to a new level on home consoles.
Sonic Adventure 2- whether the Dreamcast version or the advanced Gamecube version, this was a great game. Not traditional Sonic but Sega found a way to take Sonic past the fast paced platforming to a game with a lot more depth.
Super Mario Galaxy 2- Talk about having to live up to expectations. Super Mario Galaxy may be the best platformer ever created and to be a sequel to that is stressful. But Galaxy 2 brought new and exciting levels that offered different challenges to the already well-established style of the original. That is why Galaxy 2 might be the second greatest platformer ever created.
Now for some non-traditional sequels:
Red Dead Redemption- this took the open world sandbox genre to an entirely new level as it was able to provide great characters and great story without sacrificing the freedom found in sandbox games.
Resident Evil 4- Shinji Mikami brought video games into the modern age with RE4 and gave survivial horror games the update they required. A good mix of action, puzzles, suspense, and horror along with a control scheme that actually worked took survivial horrors from cheap scares to intense moments of survival. And to me there is only one game better than this one haha. (hope my avatar didn't give this one away haha)
