While I have the ARMS #21 on mine currently, the early version, not the current full rail, one that I have not seen mentioned is the Vltor CASV-14 rail system
. I've been contemplating switching to this for my scope mount on my M1-A. I would avoid the Springfield Armory mounts. They are not that great, been there, done that. For a traditional mount go with ARMS, Sadlak or Smith. Some like the Basset but I cannot comment on it.
Troy industries has the BattleRail
which is similar to the Vltor rail but you have to dump your EXCELLENT USGI sights in favor of a shorter sight radius.