This is so my niche. Here are my favorites, in almost no particular order (though I like the ones on top the most, getting more murky as we go down):
1. Twilight Imperium - Yes. YESSSSS.
2. Battlestar Galactica - A great game. Brings out anger when I play with some people who sometimes get stubborn and assume I "Witch hunted" them if I caught them doing something completely ludicrous and took away their titles.
3. Bang! - Best quick game. BEST.
4. Space Hulk - Two players, but the best two players.
5. Betrayal at House on the Hill - Great for getting casuals in (there really is a casual and hardcore division for board games, much like video games. Except hardcore board gamers don't get pretentious about it)
6. Resistance - My favourite Mafia-based game. A friend of mine has a boyfriend who is a troll, which sometimes kills us, though it is ocassionally hilarious.
7. Cash & Guns - Makes you feel so cool, even though you're swinging an orange gun around at people. I love finishing off my foes and even being finished off and doing a death enactment in the den.
8. Shadow Hunters - Popular amongst the crowd I often play these games with. People who don't fully understand BSG call it 'Battlestar Galactica Light'. This is in no way accurate.
9. Carcassone - Best for trolling. I rarely play to win. Instead I stack my city tiles next to other people in just such a way that I flood them with my knights and steal their points. Last time I played I got a 20-tile city which everybody had been trying to go in on.
10. Twilight Struggle - Not as intense a two-player experience as Space Hulk. Probably higher on my list if it had more players, to be honest.
My list could go on and on and on. Ohh man.
Anybody want to PBEM anything using Vassal? I've been meaning to bring this up.