First you have to ask yourself, what do you want out of your rifle? Reliability, Accuracy? Ease of takedown/cleaning? Fewer/more parts?
My choices, in order, bases on personal experience and ownership are:
#1 and #2 are a TIE between the M14/m1a and the HK-91/PTR-91/G3 (Absolutely NOT any variation of the CETME)
#3 FN-FAL
#4 AR-10/LR-308
M14/M1a- Very accurate, Proven reliability, resonably rugged and forgiving tolerances, well balanced rifle, Great iron sites with adequate adjustment-only downside is parts breakage and wear. Mags range from $15ish to $50+
HK-91/PTR-91/G3- EXTREMELY Reliable, EXREMELY Rugged, EXTREMELY Forgiving, virtually NO parts breakage or wear, kinda heavy and not as accurate as the other "Big 4" MBR's. Rear drum sight is nice and affords some adjustability but the front post is a big let down with virtually no adjustability. Brand New "Used" mags from Cheaperthandirt.com for .97 CENTS WOW!!! AVOID any variation of the Spanish CETME, Refrain from any Century Arms built guns and ABSOLUTELY AVOID ANYTHING BUILT BY FEDERAL ARMS-CRAP GUNS.
FN-FAL- Lots of Parts, Accurate, easy gas adjustment, can shoot almost any ammo, easy takedown (a la AR platform) Tolerances are fairly forgiving, somewhat heavy. Careful with parts 2 different types: Metric and Inch Pattern, with some interchangebility, but not all. Mags range from $15ish to $35ish
AR-10/LR-308- VERY ACCURATE, small parts interchangeabiity with AR15 platform, Built for Optics, same cleaning issues and tolerances of the AR 15 platform, VERY HEAVY-which is the main drawback and the one thing that it definitley does NOT have in common with is smaller .223 cousin. it is a fat bastard. There can be some ammo sensitivity but it is not platform specific and usually correctable. Magazines are $20-$50, DPMS mags only hold 19, not 20 but magpul makes a 20 rounder for $20 that is far better than the factory mags(anti-tilt and all).
Well, thats my 2 cents, but at the end of the day its kind of like buying a handgun, you really need to decide which one feel right for you and what you want the rifle to do.