in no particular order....
The Chronicles of Riddick: Escape From Butcher Bay
this game just rocks. it came out before Doom 3 and Half Life 2, but it has graphics that are more than on par with both of those titles. however, its the gameplay that takes the cake. the gameplay is all from a first-person perspective, but its more of a combination of stealth-action and FPS. in fact, there are several portions of the game where you're armed with nothing but a knife and even just your bare hands. either way, as Riddick, you're still extremely deadly as long as you remain hidden. the gameplay is top notch and quite varied, and it comes with a well done storyline. definitely one of the best.
The Elder Scrolls IV: Oblivion
yeah, you knew it was coming. what can i say?? the exploration and the varied gameplay is what puts this one over the top. it also helps that you have a number of very good storylines to help move along the gameplay whether it be saving Tamriel from Mehrunes Dagon or discovering who the traitor is in the Dark Brotherhood. beautifully done graphics, excellent sound design, and even the ease of modding all contribute to make this game as good as it is.
Rome: Total War
the Total War series was always great for combining turn-based strategy and real-time strategy, and Rome really set the bar for how to do it. the turn-based element was fairly easy to get into, and its downright addictive. the real-time based was great for the level of detail. want to send the enemy running?? hit their phalanx in the flanks with a cavalry charge. want to get them to charge prematurely?? hit them with your archers. combat varies as well depending on if you're defending or attacking, attacking a settlement or defending it, and it even varies with the type of terrain and weather conditions. there's some micromanagement involved in both elements, but its nothing very serious. again, this is just one of those games that you really have to play.
Star Wars: Knights of the Old Republic and The Sith Lords (tie)
ok, so this is really two games, but they both deserve to be mentioned at the same time. the first game had the better story, but the second one had better gameplay with quite a bit of improvements over the first one. saying anything else would be rather redundant in this forum, so i'll just leave it at that.