If you want my opinion just make sure its a company with great history and customer service. I used to think that meant US made, but 2 companies changed my mind.
GP-VapourArt
GUS Mods
Not that there arent other modders making great safe devices, but these 2 go above and beyond in their dedication to after sale help. They dont spit out a new version weeks after a launch of their supposed best of the best.... They dont do bad design upgrades because it was flawed to start with.
THey hardly ever have "problem devices" but if they do they take care of you with no hassles, no questions, no pointing the blame on the users.
The price point of these 2 is also pretty good. GUS mods offers free shipping and it gets here pretty fast, even from Europe. If not in a hurry usually 10 days? Or you can pay a bit extra and get it faster.
GP offers low priced decent shipping and if you really want it fast , for 15.00 it will be here in 3 working days, most of the time 2.
MCR from the UK, great
JM MOds great, but they are dropping their mechs and sticking to their gorgeous regulated boxes.
Metal Madness Vapors (MMV) maker of the Nemesis, great company, great customer service.
Just make sure what ever you decide on has the specs for what style you enjoy. Make sure it has venting features, not always holes, many hide the vents in the switch or top cap. Make sure it is well insulated on top cap and on switch.
Most modders will give you suggested resistance limits and to use IMR batteries only.
Hope that helps