Well, naturally I'm rather fond of the Gorn UI. I find the color scheme makes everything easy to see and is easy to stare at for extended amounts of time.
My list, from best to worst, would be:
Gorn
Romulan
Hydran
ISC
The Romulan and ISC have good color schemes that are easy to look at and use. They Hydran is good too, just a little dark.
I'd reverse Hydran and ISC, if it weren't for that ISC voice!!! As it is, I'll take the Hydrans english voice over the frogvoice.
-S'Cipio