If we're talking intermediate cartridges, I'd rule out anything in the 7.62mm Nato. This would include the beloved M14, G3 and the FN-FAL. For all practical purposes, these are 8-9 plus pounds rifles and the only advantage they enjoy over their bolt action predecessor is that they are semi-automatic. None are effective in full auto since they are far too light to be controllable.
For all practical purposes, our selection is thus narrowed down to the smaller 5.56 mm Nato, 5.45 (?) x 39mm Russian or 7.62 x 39 Russian.
With respects to accuracy, you can't beat the AR family. Gas Impingment means fewer moving parts and less to affect consistent lock-up and less variables for the rifle's harmonics. While the bugs (failure to extract, jamming) have been resolved years ago, the main weakness has yet to be addressed: poor magazine design. The AR's weakness is not in the gun but the flimsy magazine lips which, if abused, dramatically affect the AR's reliability. Compare the standard aluminum AR mag to the HK33 (HK93) magazine which is reinforced or the heavy AK magazine (you can beat someone with an AK mag).
The British L85? A repackaged AR-180 in full auto. There were a lot of problems which includes parts breakage. These should have been solved by now but I've heard some troops still don't trust it.
Reliability wise, the AK is unstoppable. Mud, sand, dirt does not deter an AK from chugging along. It does suffer from a tell tale "clack" when the safety is moved.
HK-36? I've fired it. Nifty sighting system, easy to handle either left handed or right handed.
Sorry to wimp out but I gotta think about this some more.