TF2 should always get tossed around in threads like this. its fun and can be enjoyed completely f2p, though, buying something from the mann co. store (which gives you tf2 premium) unlocks features like talking in chat etc.
also Plants vs Zombies: Garden Warfare 2 slaps so hard, and its relatively cheap (or free if you have ea play). the console versions still have a pretty alive and well community, you'll find most people go for turf takeover, team vanquish, or mixed. the pc servers are alive in american servers and lesser so in european ones, but modding has become a problem recently, if you dont mind that, id say give it a go!