Excellent question. And one a few of us are discussing right now on the Devine Tribe thread in fact.
As seems always the case, I think it depends on what you want? Dabbers are fond of very high temperatures and therefore high production rates. Using stuff like Omicron carts guys bump the temperature up and up and up looking for 'bigger clouds'.
I tend to the bottom end, where taste, not volume, drives temperature. I prefer to get more volume in parallel (larger load, larger heater) to hold the same temperature. Or take slower hits.
That said, it's easy to experiment with temperatures. For instance I just did so again using the FlowerMate pods and their 'Pro' vape that lets you dial up temperatures. I came to the same general answer as before, 390F suits me best. Others might like higher or lower, but IMO if flavor is a factor not by much. Cloud chasers would tend to the hotter end of course.
The larger issue is, I think, you don't really know the temperature you're running at. "Juice" is just not all that demanding, nothing like the precision we want with oil/wax. I suggest you can easily be 100 or more degrees from where you think you are.
We noticed that some mods (like the Invader Mini) gave good vapor set at 220F. Yes, boiling water hot. Other mods, like the Vic VTC Mini I'm now using give equivalent performance (near as 'we' can tell) set at 330F. Clearly you can't trust at least one of those?
So I rigged up a Thermocouple to try to measure things:
The Thermocouple is in under the doughnut (which is the heater) pressed against the bottom), you can see the two leads coming out. At 'the magic temperature' (VTC set to 330F) the T/C meter reads 390F, which seems to square things up for now. I hope to repeat this test using temperature indicating crayon, due this weekend. Hopefully it'll confirm the temperature (at 400F) to within 1%.
I think the heater is 60F hotter than VTC does and 170F more than Invader at that temperature?
IMO this technique (sensing resistance change in the heater to determine temperature) has some advantages in sealed heaters like the DC uses, but in open coils with wicks inside it's not so clear. In such cases, unlike sealed ones, local conditions can create hot (and cold) spots that the controller can't know about and therefore can't compensate for. Cool one end and the controller will sense an average drop and crank up the power, making the normal part (not in a draft say, or insulated by more concentrate) get too hot. Again, not an e-cig issue I think?
So, if you can really guarantee the actual temperature where the concentrate gets heated I'd say you want something like 390F. Say 370 to 430 to cover all tastes? But I don't think you know what the temperature really is, nor would anyone else. Hot spots not withstanding........
OF