Honestly, I don't like any of the valve games all that much, I don't know what's wrong with me, but they just don't click with me in general. Overall, I rank:
1. Portal (Fun, could've been longer. Didn't care for the "humor." Overall a forgettable experience. Still rank it number one though, oddly enough.)
2. Left 4 Dead (Once again, fun for a little while, but really lacks in content. Playing with friends extends the game a little bit, but only so much. Needs more campaigns.)
3. Garry's Mod (Great sandbox tool and a lot can be done with it, unfortunately I get frustrated and quit before I've accomplished anything.)
(the rest in any order)
Team Fortress 2 (Bland and lackluster. Yes I said it. I honestly don't see what's so great about this game. I don't like the art style, the weapons, etc. Basically the whole game gives me a headache.)
Day of Defeat Source (I've only tinkered around a little with this game, but honestly so many other games do the whole WWII thing better.)
Counter Strike Source (I've made several attempts to get into CS:S, but the community is so advanced by now that pretty much any n00b gets torn to shreds. I've given up playing this game frustrated so many times.)
Half-Life (I'd rank this near the top, but honestly I don't remember much from it any more. Leagues better than HL2 though.)
Half-Life 2 (Another one I don't understand. The game is downright boring. Scenery is drab and dull. Story is boring and forgetful. One of the most disappointing games I've played after hearing other's thoughts.)
Half-Life 2 Episodes (Either) (See HL2 comment above)