Ok............to answer your questions.........
Yes a Y.B. crossed into a pastel makes an AMAZING looking combo. Yes there are different grdes of Y.B. but I have seen stunning Y.B. produce not so stunning babies.....and visa versa............but apparently Y.B. can be improved upon thru Line Breeding.............
As for who has the best ones.............I DO!! ...........lol no seriously though...........
Here is a list of people that I know of who are Producing QUALITY CAPTIVE YELLOWBELLIES.....
1.) Amir Soleymani - Has also produced Pastel Yellowbellies
2.) Douglas Beard - Has also produced Pastel Yellowbellies
3.) Myself
4.) Morton Wright - Has also produced Pastel Yellowbellies
5.) The Snake Keeper
6.) Mark Bell - Has also produced Pastel Yellowbellies & a Pastel Ivory ( I believe.... )
7.) VPI
Here is a pic of the Pastel Yellowbelly from Greg Graziani's web site....


Jared Horenstein
Royal Python Reptiles
Yellowbelly Pastel