most difficult level
I'm kinda retro so I started playing this when I bought it recently. The hovercar level is uber-hard and it's only like the third level. I don't even think the nes advantage helped me.
shareLevel 9: Terra Tubes for me. There are lots of ways to get killed on this stage! It contains alot of interesting enemies too.
Then after level 9, Volkmire's Inferno (Level 7) would be next, but most gamers take the warp in Karnath's Lair and zip to Level 8. The electricity at the end section of Level 7 is horrific! In terms of difficulty that is.
Then of course, level 12 would be high up on my list too. Just an awesome level, but it's very long, and very easy to fall down or get killed.
In God WWE Trust
Level 11, the Clinger Winger, has been impossible to get through. And I only recently figured out the secret to beating level 10: try to freefall through the girders if the gaps in them are close to each other, and on the long straightaways, when the rat comes at you, try to butt him so that he bumps into the wall and goes running back and then coming at you again. Even then, there's little guarantee.
"Mark well me words, mateys: Dead men tell no tales!"