Oh, another "What's the best" debate.
The answer to the original question is simple. There is no straight answer. Unfortunately, guns, like sports teams, have their loyal fans who will never budge no matter what anyone says, often resorting to highly detail issues to support their general opinion. Not to say that the points made are not real and valid, but when you look at the big picture, they prove to be less defining then wished.
Let's have a look at some of the guns that are often held as the best.
M-1 Garand
The M-1 Garand is a large and powerful rifle that isn't as heavy or hard kicking as it's reputation suggests, but most of it's reputation about the business end is underrated.
It only holds 8 rounds but it reloads extremely fast and ammo is held in a small clip rather then a large magazine, making it easier to haul large amounts of ammo around on you. It can handle more pressure then any other battle rifle ever, accuracy is great, the cartridge is a powerful "one hiter quiter" and it handles very well.
However, it's still a heavy gun and quite large. Recoil climbs allot and is off-center which makes it hard to stay on target compared to newer guns like the M-16. A gun that is so large that it's intimidating to look at is a gun that is too large for a variety of functions, CQB being the most obvious. In short, the M-14 replaced it for a good reason, yet it remained in service along side the M-14 for a good reason.
M-14 Another gun that is far lighter and easier to haul around then it is made out to be. If I were a soldier in Viet Nam, this is the gun I would have wanted to carry. The Scout model FEELS about the same as the AR-15 in terms of weight, though I have never actually weighed the two. The M-14 is an extremely reliable gun yet extremely accurate as well, and one of the better handling rifles ever made. It fires the 7.62 NATO/.308 round, which is legendary for it's versatility.
AK-47/AK-74. This gun is both underrated AND overrated. It was built around reliability. Accuracy is good, but reliability and firepower are the main advantages. It can take a ton of beating without failure and kill just about any land animal on earth with reasonable accuracy. Those features alone make the Kalashnikov type rifle one of the best "doomsday" survival rifles in the world.
However, the AK is not without faults. It has a shorter accurate range then Most of it's rivals and it's stability is not that great. The AK-74 fixes allot of the issues with the original 1947 pattern, but not all of them.
AR-15. This gun was literally designed around stability, so it's ease of handling and accuracy are almost second to none. Only some of the newer battle rifles rival it. It's firepower is also highly underrated. Those who know ballistics know that energy is an incredibly important factor, and a .22 caliber bullet moving at over 3000fps is nothing to take lightly. On top of that, some of the loads that get in the 3000fps ballpark in full size rifles get up to 80gr and maybe even higher. That is serious firepower with some serious range potential. When you see those numbers, it's no wonder that the M-16 has been known to take on and defeat M-14s in 1000 yard iron sight matches.
As with all guns, the AR-15 is not without shortcomings. Though reliability is far better then it's reputation suggests, it is still nothing special in this department. In fact, it's reliability is average at best. Although it's smaller caliber may be sufficient for the human beings and medium game that it was designed for, it may prove to be less suitable and effective against larger and more dangerous game then it's rival, the AK.
On a personal level, when I think of best rifle, I think of what would I want to have with me in an war time survival situation, considering what my circumstances might be like in real life. With countless factors considered, I think it would be a close call between the AK-74, M-14 Scout and FAL, ammo availability being one of the most important factors.
The likely case of an SHTF situation on American soil is either war with Russia or a revolt. If there is a revolt, it's hard to say what will be most common. .223 and .308 will probably get sucked up allot and suddenly appear in short supply, but .308 probably wont get it as bad as .223. The exact same will be true if Russia were to invade. However, if they did, the ammo that will be up for grabs is what the Russians bring with them, and that will be 5.45x39.