Let me second the WOA option. I really like the 1:7 wilson barrel. Mine's got almost 4000 rounds on it from last season, and it still shot the best X-count at 600 (9x) I had all season in the last match. Another nice thing (though I'm sure you could swap out parts to do this on another upper) is the elevation knob is numbered, 25 clicks per revolution. It's really nice to be able to look at it and see if you changed from your standing to sitting zero without backing down and counting up.
Also, the Holliger's tuned RRA trigger is really nice. Definitely the best deal for an AR trigger. It's $120 for the trigger, or $30 if you send him your RRA (match, not an ordinary one) trigger. Well worth it.
All that said, the Rock River rifle isn't bad, either. And if you find a good deal on it, or the extra $100 or so for the WOA is a bit much for your budget, go for it. I like mine, but I really like the WOA upper.
I've also heard the Bushmaster's not bad. But I have used the trigger, and it's really bad. It feels nice, when there's a second stage. But sometimes the second stage disappears, then returns the next shot. It's not good at all to start taking up the slack offhand and then hear a bang. (Other folks I shoot with have had similar experiences with it)
I haven't used, or heard much about the others, so I can't comment on them, I'd be surprised if they're no good, however.