8. It really isn't hard. Of course, I still feel like Master Hand and Crazy Hand at the same time are actually slightly harder than Master Core. Master Core is laughably predictable.
8.5 was the best I've gotten. I tried 9.0, but I failed on third to last somehow because of a stupid move, and right now, I'm trying to save up enough coins or whatever those things are to do level 9.0 again. (I went from the lowest, and went up by 0.5 each time.)