Re: FN FAL
The FAL is a fine battle rifle, but an accuracy wonder it ain't. The receiver of the FAL is too long and too lightweight for the kind of match accuracy you can get out of an M-14/M1A. That's why the FAL was never really successful as a sniper/spotter rifle with any service. If you're not looking for National Match type accuracy, the FAL is a superb choice.
The G3 is an outstanding design, but the action is never truly locked, and the roller-delayed blowback has such high initial bolt speeds that you need decent quality brass in order to keep the cases intact during extraction. Even then, any G3 operator or HK91 owner will tell you that a HK roller lock rifle will mangle the living crap out of brass because of the fluted chamber. If you like to reload, the HK is not for you. As with the FAL, the G3 is capable of good accuracy (especially the G3A2 and A3 variants with free-floating barrels, as well as the HK91), but they're not thousand-yard bullseye rifles. They used to make an accurized sniper G3, the G3 SG/1, but those were just hand-picked accurate G3s fitted with 6x42 scopes and adjustable cheek pieces.
The M1 is pretty much the epitome of the Garand design, the most accurate .308 battle rifle out there, and also the most "conventional" design. I'd mortgage my eternal soul for a nice M1A National Match, but I haven't been able to set aside that kind of casual spending cash yet. They're of a different design school than the stamped-steel HKs or the FALs, but there's absolutely nothing that these rifles can't do just as well as the HK or FAL.
Tough call. For what all these rifles go for these days, I'm almost tempted to say that the M1A would be the best buy. HK91s are way too expensive, and nice pre-ban FALs are not much cheaper than a good M1A, if any.