Yugo ranks right up there. This engine was "tossed" by Fiat and was given to Yugo who made it worse than Fiat ever could. If you were lucky you could maybe make 3000k before it blew up.
The 1960 VW Type-2 engine also ranks up there. It was a 36hp engine that looked like 40 hp. It had split bearings all the way through and the thrust bearing was on the #2 main bearing saddle in the case and oil seal appeared to be about 5" in dia. This engine would lock up on the main brgs. anywhere around 6,000-20,000 K. I think VW warr. these engines to 40,000K. Folks in some circles called it the "Bastard". VW called them a "Reinforced 36hp" engine.