Actually Steve Vai is no slouch either...
You're right - he's probably even better (techinically) than Joe, but...
Something about Satch makes me have to give him the edge...so many of his tunes are so well constructed. He doesen't just 'go off' each and every time...he holds back at lot, for the sake of the song, and the songs are better for it. When he does cut loose, it seems so much more impressive, as a result.
A true musician of our modern times, if there ever was one. The fact that this stuff isn't for mass popular consumption makes it even more special to me. When I listen to Satriani, I feel like I found gold in the backyard.