Hitman: Blood Money has provided endless hours of entertainment for me. There are so many ways to do the missions, so many cool little areas of the levels to explore, and so much fun to be had creating mayhem that I never get tired of it. My personal favorite is the one where you're hired to assassinate a guy in the Witness Protection Program who is hiding out in this suburb. There's something very entertaining bringing a shotgun to that level and wandering around like the villain in a horror movie. Or bringing the sniper rifle and assassinating everyone in the neighborhood from the tree fort in the back yard.
Thief 1 and 2 also claimed hours and hours of my life, for many of the same reasons. I myself love to sneak around, getting places without people seeing me and scaring them, so I tend to like games that let me do that to an extent that I can't in real life.
I also really like the Rockstar game Bully--it's like Grand Theft Auto in high school, where the criminal gangs are replaced by cliques (the Jocks, the Nerds, the Preps, the Greasers, etc). You can get into schoolyard brawls, you can give kids wedgies, you can go into the nearby town and wreak havoc...There is never a lack of things to do in that game.
Sid Meier's Pirates, both the new version and the old one from the 80's. In terms of sheer love, this game/games is probably just about my favorite of all time. It was one of the oldest open world games ever made, and while the remake definitely improves the idea and gives you enough extra to do to make sure that you're never board, the original was about as close to perfect a game as I've ever seen!
Alpha Centauri is probably my favorite strategy game and has soaked up much of my life. I've never been that gaga over the Civilization games, but I have enjoyed many of the spin offs. I also really like the old game Colonization--it was a Civ style game (I think there might have been a remake recently) where you play as the settlers of one of the European powers establishing colonies in the New World. The game culminates when you declare independence and have to fight a revolutionary war against your former country. It's like Civ but with more focus, and that's something I really enjoy.
Another old game series I really like are the Wing Commander games. Wing Commander 2 was a big part of my childhood, but my favorite from that series is probably Wing Commander: Privateer. Another early open world (or open universe) game, you play a freelance pilot for hire. There are tons of missions in the game that you can pick and choose among, flying around the sector as you like and encountering a number of factions--the Militia, the Military, Pirates, Merchants, Mercenaries, the Kilrathi (the enemy alien race of the Wing Commander universe), etc. A marvelous game that is fortunately being reprogramed for modern computers by a group of people working for free (bless those people who undertake such awesome projects!).
I loved the Quest for Glory games. In my mind these were just about the pinnacle of adventure gaming (though the Space Quest games were pretty good, too!), especially Quest for Glory 4. The stories were marvelous, the narrative style was very compelling (humorous in a very restrained, very charming way, but with the ability to also be serious), the blending of RPG elements like stat building and combat (good for replay value, and the reason I played these games way more than games like Monkey Island, which were awesome but were only good for one or two play throughs)--these games just hit on a truly golden combination, and I'll take the memories with me for the rest of my life!
Finally, Mount and Blade: Warbands is another game I've spent countless hours on. Whether I'm playing a disgraced noble trying to build an army to reclaim my birthright, a bandit warlord spreading terror across the land, or a merchant peddling my wares and trying to dodge bandits, I absolutely love this game, so much that I have to be very careful about playing it, because it will suck up so much of my time (it's hard to play it for less than 6 or 7 hours in a sitting).
