The Gaur 21 is recommended for massive DL users for 50+ watts.
It has the mass of half a brick, it’s not pocket-friendly, but you get what you pay for.
This is true for all dual 21700 mods anyway.
For tanks, you can achieve maximum leak resistance with leak-controlled tanks
…then comes top airflow, then bottom airflow, and at the end of the line are most RDAs, from this perspective.
Except for the latter, if the wicking is adequate and the O-rings seal well, leakage is impossible. However, good wicking varies from tank to tank. There are beginner-friendly ones that can be wicked with your eyes closed… (e.g., B80 and other RBAs), but there are some that are notoriously difficult to get working 
However, most RTAs are relatively easy to figure out.
In short, that’s it.
If you have a tank in mind, ask about it here… there are experienced users for almost all of them in the group.