Of the ones I have owned or been issued (m-16A1, AR-15, AK, SKS, M-14, Hk91clone, Mini-14), the AR-series would be the favorites. I have not, of course, ever carried one into battle, so I won't go into war stories or buddy stories. Based on the individual rifles that I own or owned:
1. AR/M-16A1- own/ed 4 different AR's , 2 Colt 2 Bushie, issued an M-16A1. No relaibility problems with any. Accurate. Light. Excellent ergonomics. Optics a snap to mount. Fun.
2. SKS- bought two Norincos way back when they first started importing them. Kind of heavy for the size of cartridge it uses. Stocks, as issued, were uncomfortably short for me. Accurate, but the sights don't let one take full advantage of the accuracy potential. Optics aren't easily mounted. Fun.
3. M-14- owned a Polytech M-14 clone. No reliability or durability issues. At the range, it shot comparably well to Springfield Armory "rack grade" rifles. Sold it because it just didn't amuse me to any great degree. Made a nice profit, too. Okay, but not really fun.
4. Mini-14- owned 2, an early stainless Mini and a blue Ranch Rifle. Accuracy was, at best, mediocre from both. The Ranch Rifle showed poor fit and finish and a had a flimsy rear sight that was prone to self-destruct. Good high cap mags were a pain to find. Sold both. Not fun.
5. HK91 clone- have a PTR 91 i recently purchased. Haven't had a chance to shoot it enough to have formed any solid opinions yet. Preliminary findings are that it is reliable and accurate, though far less ergonomic than I would like. The medium/heavy weight barrel also makes it somewhat musszle heavy. Fun, so far, anyway.
6. AK-47- owned a Maadi. By far, the most egregious waste of time and money I ever engaged in with respect to guns. It was reliable, as AKs are reputed to be (so were all the other rifles I listed here) but that is really all I can say in its favor. Canted front sight wouldn't permit the rifle to be zeroed. It shot 8 inches right at 25 yards even with the front sight maximum windage adjustment! That was of small concern, really since the sights were so crude as to be minimally useful anyway. Since there was no side rail, there wasn't the option of using a Kobra or similar. The finish on the wooden parts, judging by smell, was based principally on boiled camel urine. The paint on the metal parts was already flaking as the gun came from the box. Screw heads were buggered up at the factory. Trigger slap in spades. Mechanical accuracy made the Mini-14 look like a high-end sniper rifle. Much time and money later, I had turned the shabby thing into something like a faux Valmet, but nothing could be done about the accuracy issue. Sold it, at a profit, to a guy who was enchanted with its looks. Pretty freaking far from fun.