Cheap and flexible: ego/riva/echo VV
Cheap, extremely flexible: some VV box
Expensive, quite flexible, very safe and fool proof: provari V2
Flavour wise:
Cheap and a bit fiddly: dripping on any of the above with a 306 atomizer
Cheap (sometimes) and relatively straightforward: bottom feed box + 306 atty (old goat mods are cheap)
Expensive and fiddly, but some say the best: rebuildable atomizer with tank on a VV
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Forgot to say, the best cheap alternative to a provari is perhaps the lavatube. But the batteries it comes with will hamper performance. An expensive (£10+) IMR battery will improve things considerably.
Still, both provari and even more lavatube will have problems running low resistance dual coil cartos, for reasons that you'll find online, better explained that I could.