can't really choose between dota 2 and csgo

It's dota 2

definitly Dota 2

TF2

I have a habit of coming back to tf2 monthly and it's still as fun as it was years ago.

TF2 for sure

I could probably beat you and nearly up 2v1 btw

Portal 2's co-op.

Garry's Mod is great, because you can basically do anything with it. It has a huge amount of replayability, and you can further add to it with the Steam Workshop.

Not a Valve game.

Portal co-op

TF2 is easily my favourite valve game to play. I don't play many games that aren't FPS, and for me TF2 beats any of the CS franchise pretty neatly - I prefer the much greater variety of weapons (even though I still mostly stick to hitscan when I do play, because shit pings) and the potential for much faster-paced gameplay. Coming from a quake player that's probably not surprising. :)