Warframe.
It's not perfect and I take my fair share of hiatus' but every update adds something interesting and I soon find myself falling down the rabbit hole again of shit I haven't yet done (min-maxing builds, getting the good arcanes, Rivens for weapons I like, Eido hunting, etc)
I don't get out of that hole for another 4 months or so and then it's not too far off for another game update.