The Pondorosa $2.50 in Doug's auction looks like a pretty good one - probably not the rarest though....

Lot #238. $2.50 Ponderosa. Min.: $160

Current: $460

The Riverside 1st issue LCrown $2.50 is listed as Unique so it must be pretty dang rare. The Del Webb's Primadonna snapper is an R-10.

I don't have scans of any of these - this is my best one....