Old Hagstroms had the thinnest (not narrowest, that would be a VOX) necks of anything I've tried. Also, I had a Moonstone Eclipse with a super-shallow neck, like a fingerboard with a thin maple backing.
I have an Alembic Rogue with an extremely flat radiused neck. That is really what I am asking about. Rest assured that I have been following this thread and looking for basses. I found a cheap Ibanez RD707 on ebay. I may pull the trigger on that one. Keep 'em coming though!