I don't think there is a "most" underrated.
Seriously, half the '90s emcees are underrated to the point cats don't even respect them. smh.
Rakim, sadly, underrated.
Phife Dawg underrated.
The whole Digable Planets underrated.
Lupe is both underrated and hated since the release (and pre-release drama) of LAZERS. He can trip in the interviews and beef with the labels all day. Doesn't change he's one of the illest emcees to touch a mic.
Seriously, who's got a more technical, layered flow than this man.