L4D2. I used to be a REALLY big player in TF2, did trading, got a 100 key unusual at one point from trading up, used outpost and trade.tf, all that biz
Then, I lost my unusual in a hilarious way
A scammer added me thru outpost, and tried to do the 'trusted friend' scam, in which you give your unusual to someone on your friends list to make sure it's not duped, then they give it back to 'you', but 'you' is actually one of the scammer's alts impersonating you.
So I knew he was scamming and decided to mess with him a little bit, followed along with the scam, passed it on to a buddy, ...and that buddy still fell for the scam and I lost my hat.
After that I was done. I'm still friends with that guy (we'd known eachother for eons) but I no longer play a game so badly plagued with greed.
In terms of gameplay, it's really fun (I love rocket jumping, and that demosurfing was my CAREER back in the day), but the community kills it for me.
So, I say L4D2. It's a really well-designed game, and that's all she wrote. Never played DOTA, CS:S, or CS:GO, so I can't say anything about those.
Cheeeeeers