Just with my personal vaping/rebuilding philosophy, anything below .2 ohms falls in the danger zone...
Also, a Quad coil at .2 ohms really shouldn't be near as hot as a dual coil at .2 ohms. The same amount of power is being distributed to more kanthal, thus less energy (watts) per coil. I've never done quad coils, and don't plan to. My drippers don't easily accommodate it and I have no interest.
As everyone else said, it's all dependent on your build. I believe that chamber size, drip well depth etc have far less impact on the vapor production that how you build it.
My vote goes to the Trident V2. I own two, both clones. One has been modified into a bottom feeder for my Reo, which is awesome. I'm at either .39 or .49 ohms, pretty sure it's .39 and it's crazy. Reo's, even with the updated internal contacts still have a higher voltage drop than some of the nicer mechanicals currently in existence. The same build on my Reo won't hit as hard as it does on my Stingray clone. Of course, voltage drop increases as ohms decrease.
I typically take 1-2 second pulls on my Reo and blow bigger clouds than a 16 second pull on my genesis atomizers. My RSST was running 2.4 ohms at 6 volts on my Provari and it produced sizable clouds, but I had to take a long pull for this, mostly because it took a while to spool up. With the Provari's 3.5 amp capacity, it became really limiting for me, so I converted to mechanicals. I did successfully build a 10 wrap microcoil on my Provari with 26 gauge, but my usual build at that time was a 14 wrap 28 gauge microcoil.
Sub-ohms changed that. You certainly have to brave the inherent risks and be educated on battery safety. Sounds to me like everyone here is, so not necessary to get in to that. I am very satisfied with my Reo at .39 ohms.
Be safe and enjoy the vapor!