For me the hardests are: "For The Love Of God" and "Shyboy"....
I think that all Steve Vai moves are difficult, but they are not impossible. They may sound difficult but as a matter of fact they are not.
For example: DLR's Big Trouble sounds very difficult song but after few hours practise it's not so difficult, and after few days it's easy.
I've practised "For The Love Of God" about 2 days and "Shyboy" a little longer.... these two songs are very tricky.... especially the solo sections.....
What do you think?