I bet if you did a search on this topic you could read for days.
RikWriter asks some valid questions, it all depends on what you want to do with it.
The following are my personal opinions, which most folks around here know aren't worth much.
AR: I have the most experience with this gun. They are handy, accurate, and usually reliable. There are literally tons of aftermarket parts. An AR can be configured any which way imaginable. I have a Bushmaster and I love it, however, they can malfunction, I recently lost a little faith in the design when my gun started to have a bunch of really weird malfunctions. We figured it out eventually, my gun was being jammed up by a tiny little brass shaving that had worked its way into the ejector spring. But they are usually really good guns. The accuracy of most of these is excellent, and it has the best handling features. Good sights, lots of different mounts, float tubes, bipods, flash lights, whatever you want to do with it. The .223 is a flat shooting round, but not as versitile as the other calibers you mentioned. (You can't always hunt deer sized game with it, depends on your state). But every gun nut needs one.
AK: I've had a hard time with these because they are usually stocked for people much smaller than myself, so I'm probably a little biased against them. I will however say that I have never seen one jam or malfunction. Ammo is the cheapest. Sights don't tend to be as good as the AR, and I've never seen one that was as accurate as the AR. The safety is not as quick or as easy to access. You can get good quality AKs for a lot less than an AR. Personally I want one just to have. You may want to check out the Vepr, it is a new .308 AK, and is of better finish and consturction than any AK I have ever seen.
FAL: These is the Mack Daddy of battle rifles, and will most likely be my next rifle purchase. Controls are easy to use, recoil is mild for a .308. Reliability is excellent, easy to field strip, mags are dirt cheap. Rear sight is a little rough. Be carefull with these, there are many different kinds out there, originally I thought FALs sucked because I had only shot a few that were cheap parts guns (US receivers with imported parts), that malfunctioned every couple of rounds. Then I had the chance to use another forum members good FAL, and I was hooked. It was the main battle rifle of 90 some odd countries, that speaks for itself.
M1A: Really cool gun. I would stick with the Springfields, but a buddy of mine picked up one of the original Norincos, and it has been excellent. But the ChiCom guns I have seen since haven't been as nice as his. If you are planning on shooting competition, this would probably be the way to go. Recoil is a little stiffer than the FAL (probably due to the standard style stock). Have played around with a loaded model a little bit, and it had great iron sights. Mags cost more than the FAL (about $35-$50). Besides that I don't have very much experience with these, but a lot of experienced shooters who I trust swear by them. It looks the most PC if that is of any value to you.
I started out with the AR, but as time has gone on, I think I need something bigger, as in .308. But it all depends on what you are going to use it for.
Then to make things even more confusing, there is also the mighty M1 Garand, and the G3 series. And the Bushmaster Bullpup, but I don't know jack about any of these guns.
By the way I think this is the longest post I have ever had.