ANY game that's F2P, including TF2.
They are almost always filled with hackers, who always bypass the shields and ruin the game for everyone (besides TF2, rarely see those). You can never win because they're almost in EVERY match, and if they're on your team you never get points and only they do. It's terrible.
And TF2's bad because of the annoying kids, douchebags, micspam, and by far memes. The WORST of them all are sprays. I almost always either see research or something someone got off MEMEBASE and decided "LOL DIS IS GOD MEM IL PT IT ON A SPRY!!!!!!!1!11121 LOL".
These 2 things just ruin the games for me, and if they didn't have them, it could be a good game, but they'll never leave and they'll ALWAYS BE THERE.