There are actually not that many GOOD choices. There are lots of bs "names" slapped on a lot of things but this is what is well supported.
I only recommend manual batteries (harder to damage/kill with juice leaks, easier to get a good hit every time, you can use a drip tip to drip on an atomizer to test flavors)
The models I recommend have both atomizers and cartomizers available.
100 mah is about 1 hour of moderate vaping with a standard resistance atomizer or cartomizers.
Joye 510 - 180mah, lower voltage - a bit weak but small. Sold as a 3-piece with atomizer and cartridge, most prefer to use with blank cartomizers. Not many prefilled cartos for 510.
KR808D-1 - 3.7v/3.6v - my favorite is V4L.com single logo manuals (for their long 10 second cutoff as stated on some of their batteries like the single logo manuals) in standard aka 78mm battery size. Other popular ones are the rubberized models (SmokelessImage, Bloogplanet, Halocigs G6), and the standard manufacturer gold button KR808D-1 manuals. The standard/78mm battery size makes 120mm ecig, 280mah (V4L, manufacturer) or 320mah (the rubberized models). Sold as a 2-piece, this model has plenty of prefilled cartomizers and a decent selection of blank cartos. A DSE901 standard resistance atomizer can be used on these to test new juices.
NOTE - no matter what a vendor claims, a standard size cartomizer does NOT last as long as a pack of cigs. More like 1/2 pack and even then, if you run it that dry you could singe the carto and ruin its flavor. We keep things damp with eliquid. Most can get 7-10 days use out of a cartomizer that way, using thin clear liquids.
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Then for all day batteries you go to the fat battery models like Joye Ego (3.4v), Riva 510 (at Liberty-Flights.com these are 3.7v), Kgo (many like the 1000mah ones). These are usually 510-threaded (the most popular threading) and run from 650mah to 1100mah. Note - if you stick low resistance cartomizers/attys on these, you use the power up faster. So if you vape a lot or use LR then one of these big batteries will not last all day at work.
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Then there are mods of all shapes 650mah+ which use standard industrial batteries inside, from $20 plastic box mods to $150+ steel and wood models
All you have to decide is what ecig you want first. Almost everyone buys a second model by the end of the first month of vaping. And then more later on when they want to try something different or stronger. Slim ecigs are limited to standard resistance (cool to warm) attachments. Fat models and mods can use hotter low resistance atomizers and cartomizers (heat enhances nicotine's hit). And variable voltage mods let you crank up the voltage to increase heat.
I think at minimum it's a good idea to carry more than one battery, have 2 chargers at home so you can charge both batteries at once and one charger at work. And for any ecig you like and depend on, you need spares of everything. Plus, as a new vaper, you need plenty of juice - most new vapers use juice faster than they expected and the mail is slower than they expected.