This would be a difficult decision, as I don't like to limit myself to a single genre or band.
Opeth is my favorite band, and they cover a decent range from Death metal to prog metal to psychedelic rock with a little bit of jazz fusion thrown in occasionally. Mikael Akerfeldt has also appeared as a guest musician for several other bands, and he was a member of Bloodbath for a while and also Katatonia .... which adds more death metal, as well as some deathgrind to the list of genre's.
Then there's Steven Wilson, which covers a lot more rock, and overall a greater number of songs and styles, since he's in like 10 bands. He's clearly my second favorite musician. He's also done production work for Opeth. (as well as his collaborative album with Akerfeldt)
I'd also consider Arjen Anthony Lucassen as his bands/projects generally include tons of awesome guest musicians, and he's played as a guest musician for 20+ other bands, so there'd be a good mix of music, and plenty of music to go around.
One other mention I'd consider is Dan Swano. He's been in a number of awesome bands and projects. he's done production and engineering for a number of bands, and essentially, he's had his hands on so many albums, I'd probably be able to listen to music for a week straight without ever hearing the same thing twice.
But ... I really couldn't decide between those 4.