If you mean "best" (subjective);
Maybe the "best" Lumia, but definitely not the most popular.
If you mean popularity:
Personally would guess between L920 and L1020.
L920 gained a reputation for its rigidity and the L1020 for its massive camera.
When I talk to most of my friends and mates (I usually help people buy phones), the mostly seem to know those two.
If you mean the highest selling:
WPCentral already did an article on this earlier this year in march. The highest selling was L520.
Link:
Lumia 620 supplants Lumia 920 as the second-most popular Windows Phone | Windows Phone Central
NOTE:
The title says most popular, but in actuality, it means in terms of sales.
But in the general sense of popularity, I believe its the L920, but due to the higher price, its sale isn't as high as that of the L520.