I specifically like the maps in Portal 1 better, but Portal 2 is a better game.
It's a hard pick but with the prospects of Source 2 modding seeming to be completely fucked since only fucking WINDOWS users get SDK tools and there aren't any communities to foster the children to make cool shit.
TF2's probably still the best Valve game.
Yes, even with 24/7 mental cases seething impotently that Valve isn't right now letting them take over the game and ruin it by "fixing the balance" (Removing and/or nerfing everything they ever get killed by and being allowed to ban people from playing)