Magpul MOE. The standard A2 grip swims in my hand. This one fills it up more, no finger bump, and puts my hand farther back so the trigger feels better. YMMV.
Hogue overmoulded with finger grooves and storage compartment. I really like it, comfy and has a nice ergonomic feel, the storage compartment has enough room for batteries. I really like those skeltonized ones though, but feel no need to change the Hogue as it works as intended. Never say never though
Just remember that when removing the grip, there are springs and detents that can launch if not careful and do not go gorilla on the grip screw when replacing...some people have stripped the thread in the polymer upper. Sound advice.
I use both Hogue and Ergo grips on mine. The softer rubber beaver tail on them fills the gap on the lower receiver where magpul grips have a slight space. I have also read some where that the Magpul MIAD grip is a no go because there is no support for the take down spring.
Ergo deluxe, I'm happy with it. The open space in the grip isn't capped, so if grip storage is important to you, you'll need to improvise on the deluxe model.
I put a Hogue AR grip on mine immediately after I bought it before the gun even made it to the range. I like the feel of the Hogue grip a lot, and chose it in particular after handling the 15-22 Performance Center model which comes with that grip installed from the factory.