I’ve been a fan of everything Kobold Press has released for 5e so far. Their two monster books, as well as their setting book and the accompanying player option book (that doesn’t necessarily have to be used with the setting) are my favorites.
I enjoy a lot of the work created by Benjamin Huffman, aka Sterling Vermin. He’s got several good classes (magus and pugilist) and lots of interesting races.
Dan Coleman’s “Dungeons on Demand” series is amazing, as is his Ebonclad setting/concept.
I’m also a huge fan of Mage Hand Press (sometimes calling themselves the Middle Finger of Vecna). They’ve created several brand new classes (or reinvented classes from other editions that 5e didn’t have yet) like the witch, gunslinger, warden, warmage, alchemist, gadgeteer, etc. that are just amazing, and their archetypes for existing classes are good, too. I’m looking forward to their sci-fantasy 5e book “Dark Matter” that should be releasing in a couple months.
I really like Paradigm’s “Arcanis” setting, and their mechanics ideas are good but tweak base 5e a bit to where they felt the need to release a new core book just for their version of the game. They have one of the more interesting concepts for a psionic class that I’ve seen.