Color.
As far as what specifically offers the best colors I'd say
Scolys/Chalices are probably tied for number 1. A LFS recently sold a large chalice colony for $2,300...if that tells you anything. This same store charges a flat $350 for all scolymias.
I'd say acans are a very close second. I've seen people charge $45 per polyp for rainbow acans. If you have a colony, then you're sitting on some dough!
And for that matter, anything "rainbow," consisting of at least three or more different colors, is going to cost a pretty penny and grab peoples' attention.