I personally enjoy a little bit of Happy Wars every so often. I find the game to be fun when played in small sessions. I also hear a lot of people enjoy World of Tanks. Gotham City Imposters is also free this month and is a decent Online First Person Shooter. Since there a other Free-to-Play games available you could just try out the ones you think are interesting.
Like said before, having friends help the f2p games as your teammates you're randomly put together with will be no help. Warface is a pretty good game, WoT is decent but needs team communication, Happy Wars imo experience you need good items and teamwork to do anything, WSOP is a good game but I don't like poker that much.
F2P right now is GCI and it's my favorite of the group, but it's only until BBT is available, I've bought the game twice lol