Dark Sun
Love the setting, have never seen useable rules for it. The huge amount of magic items in 3e don't mesh with rules for cannibal halflings and thri-kreen (or for Eberron halflings, for that matter), and there are all kind of wonky rules like weapon breakage as well. Then there's the essential defiling rules, which create great flavor but cause balance headaches.
Second edition was worse, with overpowered base races with no level adjustment (no kidding), druids being way better than clerics, terrible high rolled stats, confusing rules for templars and there's more, but I don't want to bore you.