1: Supreme Commander (Forged Alliance)
I cannot think of a better waste of my time. It's superior to every RTS I've ever played, and it's far less micro than any RTS I know of. Now, TBS, that's a different story... Civ games are far more strategic than SupCom, but I'll admit there's some charm to the robotic carnage that goes on in it that cannot be easily replaced.
2: CS:S (and the zombie mod)
Both belong together I think. I'm no pro, but I surprise people once in a while. Top form players who teach you tricks and act courteous, little whiny shits who think the game was made specifically for them, and everyone in-between make it something of a spectacle to behold. I love the community for that reason, both the trolls and the noobs even add a bit to the hilarity of it all. Hit boxes can annoy me, but not too much because I don't put all my eggs in this basket anyway. I like source because it's pretty and has more options, I don't think source takes anything away from 1.6, and I love when the "set-in-their-ways types" try to tell me otherwise.
3: Civilization games (2 multi gold, and 4 are my favs)
Real strategy, realistic, serious business. I love a good few hours of Civ with some strangers, it makes a boring afternoon something to look forward to.
4: SRO - SilkRoad Online
MMO with awesome potential. When it's working the way it should (No bots, guild wars, trade/thief conflicts, and treacherous players) it's an amazing experience. It's still beautifully drawn and designed, even by todays MMO standards. WOWO's art can't hold a candle to it. It's deep and consuming, and the grind-fest that it is can be alleviated by knowing people who will Plvl you or go on leveling parties with you. Nothing like a good pmob party of 8 quality players!
5: BF2
Besides the terrible support by EA, this game is a work of art. Awesome vehicular combat, teamwork is key if you want to win brutally, and being able to play with 64+ people on a server all make this game my favorite semi-massive team-based FPS. A classic.