Waframe.
For a F2P co-op shooter, it's actually pretty good. It looks sexy, does everything it can to make your guy cool, the leveling (while obviously F2P in its scope) is still pretty satisfying and compelling. Levels with 4 people start to become really challenging, as there are **** loads of guys and plenty of mini-boss type characters. Games are very easy to get into and to play through, and jumping back into the action takes just seconds.
Of all the F2P games I've started lately (Path of Exile, Planetside 2, Mechwarrior Online, League of Legends, ect....) Warframe feels like the first one that isn't a chore to keep playing.
It's not perfect, by any means. There's a lot of reuse in the random level generation, and new mods/features are perhaps dolled out a little slowly. The gunplay still feels a little weak (reminds me a bit of Borderlands 1 actually.) The wall-running and all that fanciness still needs some work.
But those are all minor detractors from an otherwise very playable co-op PvE game.