It is also mod friendly, It's meant more for single player it feels like. But there is just so much to do. The world is also handcrafted vs procedurally generated. So you get a hand built world that feels lived in and has history rather than something a algorithm spits out. It comes down to personal taste I guess. But as far as best single player goes, I would give it too Grim Dawn. Oh yea they are coming out with a new class soon and you can duel class. So the builds are insane that you can go for, like I said 350hrs on one character and I still cant beat Ultimate difficulty
Oh think POE is the only one with insane skill options, check this out.
This is the Devotion tree this is just an extra layer that you build on top of your skill tree.