White Crested Black is usually pretty popular and, in my opinion, is among the 'easiest' to breed. (Putting quotations marks around the word because breeding isn't really an easy process, and Polish can be frustrating to work with, or so I hear.) Sticking with Laced varieties (Buff, Silver, Golden, etc) can be difficult because fancier colors tend to be harder, lacing can be wrong in future offspring and poorer lacing can be tough to breed out. This is general. All breeding is hard depending upon your goal. Quality breeding has its challenges no matter the breed or variety.
That out of the way, the more popular colors are the Laced or White Crested Black/Blue, mostly because the more popular hatcheries offer them. The rarer, on the other hand, are White (doesn't sound special but I almost never see whites around), Tolbunt (very hard to find!), Chocolate, Khaki, Splash/WC Splash (in my opinion), Cuckoo (any color of cuckoo: black, blue, chocolate, etc), and really, anything frizzled. This is only my opinion, but the choices are yours.
