Too many to choose one. 265 with optional oil filter? 400 with steam holes for cooling between cylinders? soft cam lobes and lifters? timing chains that slip teeth at 70K, bending all the exhaust valves or all the intake valves or both? O-Ring valve seals? Valve springs that loose load after 20K? Starters that need shims for correct position to ring gear? Just off the top of my head...