The worst I've had (but also the one I wish I still had) an '82 Mazda RX-7. When I bought it, the clutch slipped a little bit so I knew it was going to be an issue in the near future. Had the clutch replaced and it ran great for about another 3 months, then it started slipping again. The mechanic that worked on it was good, knew the RX7s, but he could never get it right. Must have replaced the clutch at least 4 times but could never get it right. He was just about to replace the whole tranny when I wrecked it.