I have the Bushmaster XM 15 . I chose it because it offers a lot of the features I wanted and a lot of versatility for future add ons
You might want to think about what you are going to use it for. And work from there.
If you are looking for target AR, you should consider match grade. If you want an HD/ Plinker You might want to look at ones that offer rails or flat tops.
Also there is the build your own option.