[Jakoh]

If the forum post are any indicators the 1520 is hand down the most popular lumia.
Do the sales volume confirm that?

My personal opinion after using almost all the Lumia's, except 1020/820, i think 1520 is by far the best.

[Great deal]

The low end are the most popular, same as Android....

Here is the world wide breakdown and by country:

AdDuplex Blog: AdDuplex Windows Phone Statistics Report for July 2014

[Rawliglat x]

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.

[Jakoh]

THanks 2pac. LOL

If you mean "best" (subjective);
Maybe the "best" Lumia, but definitely not the most popular...