My impression:
Slim (and most recommend manuals for these):
Joye 510
KR808D-1 (Several types: original manufacturer, V4L w/ extended cutoff, rubberized longer charge version like SmokelessImage, Bloogplanet, Halocigs G6)
And just because so many rebranders use it and it's in 7-11, so many arrive here using it: RN4081 (but it's not a very well-supported model - few types of blank cartomizers available for it)
Fat battery:
#1 Joye Ego and Ego-named clones and Egos with vendor logo/name on them
#2 Riva 510 (an Ego clone and the new Eastmall ones have a bit more voltage than Ego)
#3 Other things that end in "go"
mods (in no order):
E-Power
Plastic Box
mods (inexpensive and compact)
Provari (most popular variable voltage)
Reo (most popular bottom feeder)
What is NOT most popular is any of the bs ripoff rebrander models slathered all over the internet - the ones that each claim they are the best, highest-rated, most-popular etc. What they are best at is marketing and getting links to themselves all over the web.