The funny thing about this question is that the answer is going to be specific to every different end user. Its like asking someone; what's the best gun? As I look at some of the responses, and I see thousands of rounds listed, I wonder the likelihood that one is going to actually fire that many rounds in any particular engagement. Certainly, to our troops overseas it is possible that they could go through thousands of rounds in a particular engagement, but more likely than not, they probably won't exceed the amount of ammunition they can carry on their person. For many it might be 10-12 magazines. For others it may be much less. Here in the states I approach reliability from the standpoint of a LEO-end user. Our patrol rifle operators carry the rifle loaded with one 20-round PMag and two spare 20-round Pmags, one on the belt and one on a buttstock pouch. I also carry four extra magazines loaded in a personally-purchased Level IV plate carrier loaded in my trunk box. I guess in my humble opinion if you can fire off your personal ammunition load out, flawlessly, every time, then your rifle is GTG.
LEO rifles are not fired a lot, but they are carried a lot, many times in abominable field conditions. They sit locked in a cruiser mount, through heat, cold and moisture, sometimes for weeks at a time, and are expected to be unlocked at moment's notice and function perfectly. This is not only a measure of how well your rifle is made, but also, how well you take care of it. Even the best piece of equipment is eventually going to fail if you continually expose it to harsh conditions and ignore the maintenance. I'm not talking about old military nonsense "white-glove" maintenance, but simple lubrication and occasional wipedowns, especially if your rifle has been exposed to the elements. So, for me at least, true reliability is not a measure of how many magazines I can pound through my rifle before its fails, but how likely is it to work when I rip it out of the mount and its been sitting in my 17 degree cruiser every night for two weeks.
And, just in case anyone cares, my department issues S&W M&P-15's, and, as a rifle instructor, I've put about 6000 rounds through my rifle without a single malfunction, with a good cleaning about every 500 rounds.

