dude, this is a loaded question if I ever saw one!
Personally, I own a DPMS 308 and I love it.
DPMS is considerably cheaper than your other choices, but the good mags are expensive and hard to find (this problem is supposedly being taken care of by the end of Oct)
Armalite is the most versatile and there's a whole lot more 'stuff' available for it, but from posts I've read here, they have some issues with reliability. (so does EVERY manufacturer, including my DPMS, so take that with a grain of salt)
Bushmaster uses easy to find FAL mags but costs WAAAAYYYY too much (IMO) for the initial purchase.
If I had it to do over again, I would probably have gotten an Armalite AR10T A4 just because of the 'stuff' that I can't find for my DPMS, but I'm not going to sell my DPMS anytime soon--it's just too damn accurate!