Recommend the M&P 15-22 100% - I would avoid the GSG's, Colts etc as they are not practical as the internals are pretty flimsy - there is not much good said on forums about them.
The 15-22's polymer is very robust (incl. picatinny top scope rail section) - I have a scope firmly mounted and occasionally take it off to use the magpul open sights or a red dot - the rail is holding up well with no crimping issues if thats what you mean. Having been on the S&W US forum for a while I have not heard anyone complain about the polymer or any mounting issues for that matter - and those boys are always swapping things out.
I've had mine (15-22 MOE FDE) for just over 3 years now and love it - the S&W comes apart as closely to a normal AR as it gets so it breaks down and cleans very easily - this also means you can customise it
Added a few bits to mine - Magpul STR stock, CMC Flatblade FCG, LaRue Tactical grip, Magpul Angled Fore Grip, ambi safety, ambi mag release, ambi charging handle and some Magpul FDE rail covers. The barrel shroud I machined myself as they can't be posted here due to ITAR regulations, but it was pretty straight forward as thats what I used to do in a previous life! Only other bit I fancy would be an ACOG, although thats a bit OTT!
The target was 10 shots @ 50yds once zeroed - the rifle loves CCI AR Tactical (so does my wallet) although it shoots tighter with RWS Semi-Auto - it will rabbit well with winchester 40g subs and moderator, with only the bolt noise being heard in front of the gun. Only issue with ammo I've heard about is if folk have tried the 42g winchester subs (new ones) - case length is slightly longer so it won't cycle correctly. Mine didn't like stingers - to hot for the twist I think.
All in all this rifle is great quality and great fun - you will not regret it - my RFD has sold dozens and dozens of these over the last 4-5 years and I have never seen one back in or indeed 2nd hand! Get one and enjoy