I highly suggest Warframe. I have 2000+ hours in it, super addictive super fun, super active, tons of updates and content from the devs, not to mention they just dropped their biggest update ever, better then a squeal, all for free.
Just two things to note:
1. Do the Natah, Second Dream, and War Within quests ASAP, and do not look up anything Warframe online until you do, they add some features to the game that are so often used that it's impossible to avoid and its super SUPER story spoilers
2. Warframe is not pay to win, it's pay to be impatient. anything you can get in game that isn't completely cosmetic can be gotten from farming and crafting. Also, the premium currency, Platinum, can be traded between players for goods. So you can even farm FOR the premium currency.