It depends how you define “best”; by most definitions, it very much is the best, simultaneously across a wide range of categories (I cover its one caveat below too):

Modern variants of Stoner’s design happen to be the lightest weight, smoothest recoiling, most accurate (by a factor of 2-10x), most upgradeable and customizable, and arguably most ergonomic rifles in the the world right now — with no exceptions that I know of. (Incidentally, this is why the AR15 is the single most popular owned firearm in America.) Please feel free to provide any counterexamples in these categories; I don’t really know of any, and I’ve studied this quite a bit.

Most of its excellent performance is due to the direct impingement operating system, which as others have described keep the recoiling mass inline with the barrel (which means the recoil is pretty much entirely linear without any torque, which is virtually impossible to achieve with durable piston designs).

This unique gas operating system design also minimizes the overall weight, because the DI’s compact design eliminates the need for a separate piston / op rod assembly (which in piston operated military designs, must be fairly heavy duty, or else they will break rather rapidly from the violent forces involved). Some say it also explains the extremely high accuracy of the design, by avoiding a heavy reciprocating mass from moving alongside the barrel with each shot, as in a gas piston design.

Thus, this inherently light weight design permits you to choose either (1) a lightweight barrel yielding an overall weight significantly less than any of its competitors, or (2) use heavier and longer barrel which, despite yielding an overall weight matching the “lightweight” version of competitors, far, far exceeds their accuracy capabilities.

It really is a remarkably well engineered system, when you realize that most of the early reliability issues were due to feeding it the wrong ammo (which it was not designed for) and other easily avoidable embarrassing mistakes of mismanagement.

The only real disadvantage again comes from the direct impingement system, which causes the operating mechanism to get dirty and fouled up with burnt powder residue really really fast, whereas gas piston designs (used by virtually all other guns) pretty much stay clean and smoothly running even if you abuse them by never cleaning or maintaining them. There is something to be said for that, especially for military purposes: that is for example one reason why the AK was so successful; it just works. You can and should maintain it, yet if you don’t, it will still work reliably.

So this is just about the only area in which other guns beat out the M16/M4/AR15; while it’s reliability is pretty much on par with any competitor, this remains true only as long as it’s kept cleaned and oiled etc. fairly regularly, especially if it’s used in dirt dusty environments.

That said, the difference may not be as extreme as pop culture makes it out to be: A quality built M16/M4/AR15 will keep functioning for probably at least 5k rounds without any cleaning, before it starts jamming, and some extremely high quality civilian models have been known to go far longer than this in extreme “torture tests”.