Hard to say. What I usually recommend
Best/stronest slim ecig: KR808D-1 w/ MANUAL batteries (harder to kill, easier to get a good hit)
Vapor4Life - allows longer drags, not rubberized, manuals not as well sealed, 3.7v & hotter fresh off the charger OR SmokelessImage/Bloogplanet/Halocigs G6 - rubberized, 3.6v regulated, 40mah longer charge per size, but a 5-second cutoff (may cut off long drags, espeically since wth manuals you press hte button 1-2 seconds early to have vapor ready when you start dragging). I use both. I think the cartomizers of the latter 3 are a bit better but found my 2 main juices at V4L. Lots of prefilled and plenty of blank cartomizers for this model since it is a 2-piece. On a manual battery use a standard resistance DSE901 atomizer and drip tip to try new juices. Only put juices you know you like in cartomizers.
More charge more power
A 510-threaded fat battery model Joye Ego (3.4v) or Riva 510 kit from Liberty-Flights.com (3.7v) - use a drip tip to test juices, use blank cartomizers for your favorites. These are over 450mah so they can optionally use the hotter LR low resistance atomizers and cartomizers
The it-must-look-like-a cig
Anything the same thickness as a cig is annoying inflexible crap. You have to accept 9.2mm diameter (about the same as KR808D-1). Joye 510 with manual batteries and use Boge standard resistance Boge cartomizers to look like a cig. NiteLiteVapor.us sells the Boges cartomizers in white, tan and the normal black.
All need eliquid, especially the second 2 which have almost no decent prefilled cartomizers. Blanks are filled and kept damp. Prefills are kept damp. Everything uses eliquid unless you are rich - then you can just toss 2-3 prefilled cartos per day.
100mah of battery charge is about 1 hour of moderate vaping w/ standard resistance atomizers or cartomizers, less if low resistance attys or cartos are used (only big fat battery models and mods can use the hotter low resistance stuff)