Worst engines ever.
I worked in a machine shop for over ten years. I saw the worst of everything.
1. Mopar 2.2's are horrible. Every cylinder head is cracked or will be. It's not a question of if, but a question of when & how bad.
2. Anything VW, but especially the water boxers and the Type 2/Type 4//914 engines. Again, not a question of if the head is cracked, but when & how bad. The cracks are soon followed by the exhaust seats falling out, then Kaboom!
3. Mitsubishi 2.6 (same as Isuzu Trooper)
4. 80's Hyundai 4 cyl (esentially a scaled down version of the Mitsubishi 2.6)
5. Anything Jaguar
6. BMW had it's share of turds, too. When I left the shop in 1999, we were resurfacing 5 to 7 BMW 6 cyl heads a week for dealerships. They had a massive problem with blown head gaskets in the mid to late 90's 6 cyls. Also, the earlier 4 cyl stuff were prone to cracking cyl heads.
7. Mercedes 280 SL 6 cyl - crappy short guides that wear out, thin flexible heads that warp easily and an inefficient chamber that makes the suck fuel and spew unburned hydrocarbons.
That's a short list. I'm sure I can think of more if I try.