Daewoo K2. Main problem is that most of them that were imported into the U.S. have 1-in-12 barrels, which kind of limits them to the original 55 gr. bullet loading. If you can get one with the 1-in-7 barrel, go for it.
This was a piston gun way ahead of its time, combining features of the M16, the AK47, and the FAL. Unlike the Armalite AR-180, this one uses standard M16 magazines.