I second the Springfield Armory Greek rifles, the SAR 3 (pre-ban) and SAR-8 (post ban) with the steel receivers as being the best clone, by far.
The Portugese rifles, those assembled in Portugal, not parts guns built over here using a Portugese (FMP) receiver and shot to hell parts, are a very close second to the Springfield guns though. The Portugese guns are marked G3S (preban) and XG3S (postban), and are phosphated (parkerized) rather than painted liket he HK and Springfield rifles were.
I've owned several real HK91's and several Springfield Armory SAR-3's and the quality is comparable.