The M&P 15-22 is a strong contender because of its low cost (my dad picked one up locally in TN for $399), and because the bolt hold open works like a real AR-15. It's also lighter than a Spike's upper setup, so the recoil should be at least a little closer to the real thing.
If you don't care about AR-15 commonality, I think the Sig 522 is a great choice. It uses common magazines (Black Dog Machine) that are cheaper than the S&Ws. Pretty much all reports I have heard have been positive.
I've got a Spike's ST-22 upper and I really, really like it. Kind of expensive for what it is now that the dedicated .22 guns have started hitting the market, though.