Team Fortress 2 is probably one of my favourite all time PC game, I had so much fun playing the vast of classes of mercenaries you can pick! From Offensive to Support. It's never a dull moment playing.

You can get so many weapons, items, cosmetics and importantly...hats! Alots of hats! You can also vote for new maps or weapons to get it into the game on the Steam Workshop
The game may got hackers and bots but this game will live on many more years. Hope you pick it up from the Steam, Its free to all!