After the first few it becomes pretty tricky, because it feels like a question of potential more than the quality of their recent output. Top three would be Arkane, id and Playground for me, in some order. Rare, Obisidan and Ninja Theory are probably the three that I'm really quite excited about for the immediate future. With a lot of the devs, like BGS or 343i or The Coalition or Turn 10, they've got the IP and the funding to generate great games, but their recent output has been so underwhelming. There's also a good few where it's just a total unknown.
I hope that within five years time the absurd potential of this developer lineup has turned into something truly tangible.