Boy, that's a tough question as there are so many variables, such as how many coils, what ejuice, high or low voltage, and what type of draw is your personal preference? I've been vaping for gosh, probably 6 or 7 years now, however I'm pretty new at RDA's so I wouldn't say I'm at all qualified to give advice. However, with that being said I have read quite a bit and from what I gather, a smaller chamber can contribute to better flavor, as well as a good air draw. Some seem to swear by the bottom fed air draw such as the vision eternity, A7, or what I just ordered, an Achilles. Other small chambered rda's that seem to get good reviews for flavor (and there seem to be a ton of them), would be ones like the cyclone, or magma I think it is? Also, one I have tried and can vouch for, the less than $10 IGO-L seems hard to beat for the price, though it is a side air draw.
It's probably best to read around though, try to find people that have similar vaping habits as you and follow their advice would be my best suggestion.
Hope this helps