Metallicube on 29 December 2010
I tend to do the worst with the minecart/barrel levels, particlarly the barrel level in world 8. That level is just ridiculous, especially if you want to try and get the letters and puzzle piece. You're guaranteed to burn through at least 20 lives. I just prefer to have solid ground below my feet when I work my platforming skills.
Also, 7-k (the factory level) was a royal pain in the ass. Things get a lot tougher when there are no save points.
