I voted Garry's Mod. It's a hell of a lot of fun.
Others I'd pick are (ignoring games you've already partly played):
Mass Effect 1 (Feels kind of like Knights of The Old Republic to me. I think it's the same game engine, but completely realtime) ME2 was fun, but it pissed me right off. Everything was simplified and consolised right down to nothing. No more grenades, no more modding weapons and armor, no more collecting guns, combat is reduced to "Shoot, stay in cover till your shield recharges, repeat", No more booting about in the APC, no more heat-limited weapons with unlimited ammo. (Yeah, in ME2 those are "thermal" clips. You're ejecting heatsinks not empty ammo clips. Riiiiiiight. Maybe I could believe it if it functioned like the first game after you ran out of clips) So, I guess ME2 is worth your time if you end up liking the story of ME1 and want to see where it goes. Just don't expect the same depth of gameplay. It's a decent game, but it doesn't feel like I thought ME2 should. That said, I'm planning on getting ME3 because I want to know how it ends, hoping that they don't dumb it down any further.
Battlefield Bad Company 2
The singleplayer was pretty standard FPS fare. The multiplayer is essentially a consolised Battlefield 2 experience, with destructable buildings. MP is where it is for this game. Lots of fun, but it still suffers from the battlefield series's downfalls: If your team isn't working together, it simply is no fun at all, prepare for the meatgrinder. If your team IS working together, however,
The rest of these are in no particular order.
COD:MW2 had an alright SP campaign. Pretty formulaic and it's completly scripted, but it was a good waste of a few hours if you can ignore the CODisms
Far Cry was a blast, back in the day. I think it might lose some of its charm if you're playing it for the first time now, but it's still worth a playthrough I think.
Just Cause 2 is an absolute blast, but it does get extremely repetetive very quick. My advice on this one is play it in short bursts between other stuff so you don't get bored of the action before the end.