Click the 'Members' button in the control center at the top of the home page. (Guests don't have this button and can't see the Member lists...)
When the Member list comes up, you can sort it by any of the column headers. Just click on the word "Posts" and you'll get a list in order of post count.
The current Top 10 are:
Raoul Duke 375
cheekygal 308
George 278
Lone Traveller 239
AMonk 190
Missi 168
Vegemite 119
dragonsaver 118
Con ate dog 118
contemporarydog 116
However, I've become extremely leery of post counts. High post counts with little actual content are not necessarily a good thing.