If you count the number of resellers, I'd have to guess the Hanlin V3, but I haven't tried to count them. (An exercise for the interested student...)
The Netronix EB6xx is pretty popular with resellers, but they use about four or six similar designs that all fit the same case. You have to open them up to check which is used where. Bookeen alone has used two or three of them, all called Gen3 (+ Gold).
If you want to know who sold the most readers, it's obviously Kindle... just ask them
