Dota 2 in my opinion, cuz its free to play and u can get items with no money
Tf2 has a great dropping system (even if youre idle you can get weapons)
Nah, you can't idle for weapons or hats in TF2 anymore. It's a shame. I really like Gmod. Does that count as a Valve game? If not, the L4D series all the way.