Are we talking mainstream underrated here? Cause in that case I'd say the entire genre of Goa Trance is underrated. However, within the genre, there is definitely a difference in fame, recognition, and critical acclaim between the DJ's.
I guess if you're talking about mainstream underrated and artistic underrated I think Belgian artists have it toughest in general. Novastar, Hooverphonic and dEUS are great bands, but hugely underrated in every sense of the word.