KotOR II, for bringing a really dark and interesting set of characters and world to a universe that has always been very up beet or happy, except for its second arc which still wasn't that bad.
Morrwind, because thats exactly how every sand box should be done. Huge, tons of people with different needs, 14 bajillion cultures, beliefs, factions, etc... You can play the game for a hundred hours and still have things to do. A shame its combat system was total trash
Wc3 for RTS. Applying a well done rpg system to a fun and unique set of races that were all different, while having an engaging story that told the tale of an epic journey, tons of different heroes, and the entire life style of several races. Plus DotA is the best game ever.
GTA san andreas was pretty good to, but I think morrwind did the whole sand box thing better.