Thought I would throw some things out there to get some discussion started.
Here are RDA's I have tried that you can still find fairly easily, with a brief note on MTL usefulness. Please keep in mind that this is my opinion only. I do not speak for their suitability for other people:
TOBH: can be closed down to tighten the draw. My experience is that they tend to prefer coils under 0.7Ω. Atty gets hot fairly quickly. Not conducive to my vaping style which is warm, dense flavor. 22mm medium height
Magma: can be closed all the way down so you cannot hardly draw air through at all. I had to open this up all the way to get a good balance of air and flavor. Tends to leak if you tilt your mod to vape (as most do) and leak from condensation. 22mm medium height
Veritas: Nice, tight draw, air intake is designed to force air through the coils before it can be drawn out the drip tip. Deep well holds a lot of juice. Build options include vertical or horizontal coils, single or dual. It is a bit tricky to build on but the coils seem to last longer in it than the previous mentioned RDA's so you don't have to do it as often. I have seen people experience great success using a vast array of wicking options with it and enjoy their vape. I personally tend to do a single coil on it. Does its best 0.6Ω and lower. 22mm tall
Marquis: Considered a slightly improved Veritas. Much easier to build on, flavor slightly better, prefers 0.4Ω and lower. 22mm tall
REMatty: Tricky to build. Flavor and vapor is very good when you get it dialed in with your build. You can close it off completely where no air will get through. Flavor is not good for me in single coil mode. It excels with tobacco's which like heat. 22mm shorty
Derringer: This atty is super easy to build on. I have done everything from single to dual coils, 0.4Ω to 1.2Ω and received outstanding results. In use daily. Good flavor, good vapor. 22mm shorty
Stumpy: I was very surprised by this RDA. I was relatively certain I had found my final atty with the Derringer. The flavor from the Stumpy is outstanding, vapor about the same as the Derringer. Top airflow, capable of dual coils but does so well with a single coil I no longer build duals on it. This will also do extraordinarily well with temperature control. I just ordered 2 more. 22mm shorty
Le Magister: unique appearance. Inside the cap is several reducing chambers when really concentrate vapor and flavor. Top air flow that can be adjusted without removing the cap. I have it on the smallest hole, half-closed. It's a keeper on a daily mod. 22mm tall
Nectar Nano: This was my favorite atty until I got the Stumpy. The flavor and vapor are great out of this. I have only run this in single coil. The resevoir is great for such a little atty. 22mm
Freakshow Mini: This is similar in vape quality to the Nectar Nano, with slightly better flavor in dual coil mode. Likes builds below 0.6Ω. 22mm
RM2: ceramic deck is great for people who are new to coil building as it keeps the coil from shorting if too low. Build with the coil close to the deck for a warm vape, raise it just below the top of the posts for a bit more vapor and heat. Single coil. 14mm
Cyclone: Flavor is better than the RM2. Single coil. You can purchase an optional AFC cap which lets you allow for more air. Most MTL hitters prefer it without the optional AFC which can be closed down. 14mm
Hornet: single coil, tiny deck and designed for coils above 1.0Ω. Because this is so small, can produce a lot of heat if you chain vape. I have one with the air hole drilled out slightly and one standard. Prefer the enlarged air hole as it helps reduce the heat. 13mm
A7: by far the smallest RDA I have ever used. Tricky to get a build in there without shorting out. Gets hot quick, flavor is good. 12mm