I'm a huge fan of The Elder Scrolls III: Morrowind. =D The scale of the area/places you can visit is amazing- I have even higher expectations for The Elder Scrolls IV: Oblivion, coming out later this year for the Xbox 360. @_@
Why? The graphics *_*, the world in Oblivion shall be four times larger than the world in Morrowind (thankfully though, they're making travel much faster, if you wish to cut out a "little" but of exploring or have to get somewhere quickly), there will be far more pieces of armor, weapons, and other items, new/more guilds (one of the new ones you can join is The Dark Brotherhood =D), new gameplay (pretty much- I hear someone from the Thief: Deadly Shadows team is helping with the stealth part of the gameplay, which is an awesome game in the stealth genre).
The gameplay will be different for more reasons than this, but what I find very appealing is you can now choose one (if you wish) of the three combat/gameplay types instead of having to pretty much use all three like in Morrowind (in other words if you choose to be a thief, assassin, etc., you can just use stealth tactics/attacks/etc. instead of having to also use brute strength or magic). Though you still can use just one, two, or all three of those options, I think it will add more to the game for those who just want to be one of those.