Path of Exile is the best with the caveat that I'm in the beta for v3.0 and it makes a lot of great improvements. The new mini-map alone would make it hard for me to go back to the current live version. So I'd strongly recommend playing it, but wait for v3.0 to officially come out. It's FTP and all paying can get you as far as gameplay advantages are more (and better organized) stash tabs so it's very playable without spending a cent. Alternatively, if you buy the $30 bundle right now, you get $25 in store credit (which is plenty to buy all the specialty stash tabs), some visual stuff, and beta access.
Next to Path of Exile, my favorite is Grim Dawn (on Steam). It's Titan Quest's successor with an undead/lovecraft theme. Currently half off on Steam ($12.50).