The contraversial Kayfun V4 is what i use daily. And i don't use anything else. A half decent "dual battery" box mod wouldn't go a miss, with two Sony VTC4 18650
batteries and maybe a few spares for when those need charging (so a decent charger as well - "XTAR", "NIGHTCORE", something branded and reputable.
The component which matters most is the vapourizer. My authentic Kayfun V4 gives a throat hit unlike any other
tank i've ever owned. For me throat hit tops the list of requirements before flavour, aestetics, usability etc.
You can find a Kayfun V4 clone for very little. Lots of vendors sell them. It's not a new to the market device. Though there could be plenty of other recommendations which will come your way too. And because the Kayfun V4 is over engineered some people tend to slate it. But it does work and it does what a lot of RTA's don't do - "gives a great throat hit". Everyone wants the latest thing, and the fan boys piss all over a recommendation if it conflicts with the latest most newly released piece of crap. The V4 aint that !, it works perfectly, just over engineered that's all. An authentic will cost well over $100 if you can find one.
svoemesto, the company who made them stopped production of the V4 some thime ago now.
I hope you make the right choice for you.