KR808D-1 is the most popular 2piece
but
V2 charges too high a price for KR808D-1 parts and also exagerrates calling them 4.2v (all 3.7v batteries are briefly 4.2v right off the charger but they are called 3.7v batteries, not 4.2v)
Vapor4Life is most like the V2 model - i.e. like the original manufacturer model. But Vapor4Life adds a 10-13 second cutoff instead of 5 seconds. The manuals have a normal size button and still have a hole in the end though it looks blocked inside. Vapor4Life cartos are hot and give more hit, though not as big a drag as easily as the others below. They can use each others cartos.
SmokelessImage, Bloogplanet, Halocigs G6 are a new style - I think they are all rubberized and are regulated 3.6v instead of 3.7 but give 40mah more charge for each size. Their manuals are more totally sealed, have a small partially recessed button (a bit harder to press), and they have a 5-second cutoff. The Bloog/SI cartos are bottom coils - a bit cooler but give a bigger drag easier.
Most people use and recommend manuals. It is easier to get a good hit on manuals.