the x-com series is the best game compared to it's time, it's like civ + tactics.
other games good to their time is civ 2, and starcraft.
These games aren't the best, but compared to their time they are groundbreaking and amazing.
I would say rome total war right now just because of the massive size, beauty and tactics involved in it. If you are into planes i would definately recomend the anniversary editionb of flight simulator, its a simulator and not much of a game though, be warned.
half life 2 was just mediocre imo, i know a bunch of one minded people will go apeshit on me for this but half life 2 is just the PCs Halo, plain ol high graphic FPS with nothing new. Just like the other FPS it can be cleared on highest difficulty with my eyes closed.
Other good games, Battlefield 2, CS, AOE III, oh and I have to plug this guy as much as possible because he's an entrepreneur, A tale in the desert