The game I've spent the most time playing is Diablo 2. The game I've had the most FUN playing is probably Smash Bros. Brawl. The most beautiful game I've played is Crysis. The scariest was Resident Evil 2, or maybe Dead Space. The best bosses are God of War 3. The best sandbox chaos is probably Prototype. The game that made the biggest and most lasting impact on me is probably the first Metal Gear Solid.
Overall, probably Smash. So many great memories, it's the ultimate multiplayer game. But for single player action, I really dig the Metal Gear games. I know there's way too much goddamn dialogue and the lack of a freaking functioning jump button is terribly annoying, but Jesus H Christ - some of the sequences in MGS4 are just so balls-out. The final fistfight with Liquid is just freaking ridiculous, I've never seen a final fight with that kind of emotional impact.
Half Life 2 is nice too, but MGS, in my opinion, makes the most effort to advance gaming as an art. Is that really the goal, or is FUN the goal? I don't know, but those games impressed me and made an impact.