Leupold Mark 4 16X on a Basset mount.
I think it's a good compromise between size, magnification and field of view.
I do NOT think a more powerful scope would make the platform more accurate.
From my experiences, it's not a sub-moa platform and is NOT capable of pinpoint accuracy no matter how much you spend on scope and mount.
A Savage 110 or Rem 700 is MUCH more accurate, imho.
Flame me if you want..but my Rem 700 VSSF outshoots my SAI and LRB Tweaked Platforms every day...with stock ammo versus tuned handloads.
The M1A is a decent 1.5 MOA gun that can be improved with a lot of money and excellent gunsmithing ..but for the money..the extra money on a super 'Scope-Mount is not worth it.
I can get 3-4 shot-touching rounds with the M14's, but then I get a flyer or 2 no matter how careful I am and I am convinced it's a great battle gun, but sucks as a pinpoint precision shooter....which it was never designed to be.