Without wanting to sound evasive, it entirely depends on what you define as "popular".
e.g.:
Most sales
Most hand regged
Highest sale price(s)
Highest average sale price
Highest ROI
Highest increase in total sales
Highest increase in sales price
Which keywords
All the above based on "this year only" versus "all time" and other considerations
etc
As you are asking the question, one thing I will say to you is stick with good keyword .coms until you understand the value of the other extensions. You could use namebio.com and play with the search form, choosing different extensions and sorting by price, then date (etc). Try adding some popular keywords, and see which extensions sell popular keywords and for what price.