The standard answers will be KR808D-1(808) or 510/Riva/eGo. These represent two different batteries, with different connectors.
The 808 family is what I use. It's 3.7V output, which for a given resistance atomizer (the thing that heats the liquid and makes the vapor) will produce more power, more vapor, than the 510 family. It's normally used as a two piece -- the battery and what looks like a cig filter, a cartomimzer/carto, a combined cartridge holding the liquid plus atomizer.
The standard 808 battery is 250 - 280 mAh (the current rating), seems to give me about 2-1/2 - 3 hours of moderate use between charges. It plus carto is about the length of a 100mm cigarette. The XL (or Mega, in NHaler terminology) 808 battery is 380 mAh, noticeably longer time between charges, about 1" longer than a standard battery. NHaker also carries a Greenworld G400, a 900 mAh battery that really runs a quite long time between recharging; this battery is a bit thicker than the other two, and length is in between a standard and an XL, and it has a black matte rubbery finish (I find it nice to hold).
There are both automatic (auto) and manual batteries. I've tried both, use auto most of the time -- just have one manual passthrough (PT) at work right now. The current 808 autos at least seem pretty decent, I find it more natural to use them than to push a little button to get vapor.
The 510/Rvia/eGo family are also quite popular. 510 is a smallish battery, supposedly not great run time but many like it; you can get a personal charging case (PCC) for it, same for the 808, to charge it on the go, while away from a power source. eGo is a bigger (physically) 510 family vaporizer, still 3.2V usable output, battery size anywhere from around 650 to around 1000 mAh. Riva is a lower priced large battery variant. These units are generally used as a three piece, seoarate cartridge and atomizer, though there are more cartos now coming out for them.
Me, I went 808, quickly added some XL/Mega batteries, bought a set of G400 batteries after 90 days smoke free. I really like the G400, keep standard 808 stuff around for backups or loaners for people to try. I've added a couple of passthroughs (basically a connector at one end and the other end has a USB connector that you plug into a power source), one in my car and one at work.
As I said elsewhere, NHaler and vapor4life are my main sources for hardware, though I've had good experiences also with analogalternative and with vaporsetc (the place that's out of Rochester).
Good luck!