I'll cast another vote for the M1A.
The AR-10 may be more accurate, in general, and will be easier to scope. But these rifles are also going to be generally less reliable, and there are so many different designs out there, you can't be sure parts or even mags are going to be interchangeable.
The FAL may, possibly, be more reliable. But it won't be as accurate, and what's more, it will be more difficult to shoot to its potential due to its inferior sights and trigger. The M1A's sights and trigger make it a dream to shoot, and almost effortless to hit with, in my experience. And while I haven't torture tested my baby, my M1A has only had two malfunctions in the 1200 rounds or so I've put through it--both attributed to the same mag, which I marked and discarded. I've had no issues since. Carried the rifle around the woods this hunting season so it saw plenty of inclement weather. Didn't seem to notice. Funny, my grandpa said the M1 Garand didn't seem to mind the snow and ice it saw in Korea either.
I don't understand people's affection for the HK roller locks, but then, I've never shot one. Handling them, they offer the worst ergonomics and balance of the four, by far. It's the only one that doesn't offer a BHO. Among those who have shot all the designs mentioned, it is almost unanimously agreed that the roller lock has the most severe recoil. It takes far more attention than any of the others to get "shootable," that is, you don't have to give an M1A or AR a trigger job, nor have the goofy mag release changed back to something you can use. Then again, you don't have to modify your any of the other rifles to get them not to chew up your brass. And do you really want to compare the durability of the only stamped sheet metal receiver in the group to any of the others?
If mounting optics is a possibility, give Springfield's Cluster Rail System a look. It is the same rail made by Vltor and distributed exclusively through SA that they include on their SOCOM II rifles. While it can make the rifle muzzle heavy, this is true of most rail systems on most rifles, be they ARs, AKs, or otherwise. It provides a low profile, uninterrupted rail at the 12 o' clock that is 17 inches or so in length and runs from the stripper clip guide to the front of the stock--making a rifle so equipped easier to scope than probably many ARs. You may still have to select ring height working around the rear sight, and you may still have to get a cheek piece to adjust your comb height, but that's neither difficult nor expensive. The Cluster Rail also provides shorter rails at the 3 and 9 o' clock for accessories like lights and lasers, and a removable bottom rail for attaching a VFG or bipod. If Springfield will install this rail system on my M1A, I think I might have to have them--I've looked and this is by far the best thing going for optics mounts on an M1A, without dropping it into a Sage or other chassis system. It's also low enough to allow use of irons with optics removed.
If you're looking for sub-MOA accuracy right out of the box, you may want to get an AR-10. But if the 1.5 to 2 MOA accuracy an OTB M1A is going to offer with cheap 147 gr milsurp will suffice, the M1A is where it is at. It's an American icon and true "rifleman's rifle," and I have quickly grown very fond of mine...