They are all high quality AR15s.  There are minor differences, but the bottom line it that all the parts will interchange, assuming identical models to start.
Each brand has its adherants, but it's more of an academic debate, IMHO.
For example, I doubt that a Bushmaster fan would turn down either of the other two if they were offered at a good price.
There are other good brands, and a couple of bad ones, too.  Check out the Industry Forums on this site, and see what people who own them have to say.