I would also guess that Hugh Alison is going to be on that list, as will Perry Maxwell, likewise Wayne Stiles. If we're going obscure, I'll guess that Vernon Macan gets a nod.
For my money, its definitely Tom Simpson.
But underrated is a relative term, and given his audience, I would not be surprised to see William Flynn in there or Harry Colt. Colt, Braid and OTM are the most underappreciated architects of all time, relative to the contribution of each to the development of the aesthetic.