The REO is the most flexible mod on the planet. It's dripping upside down, from the bottom. It was designed in the days before RBAs got so popular and so has a juice well to catch any juice from disposable atomizer or cartomizer air holes. Either work well on it, bridge-free disposable attys particularly so. The relatively inexpensive Reomizer 2 is a good, leak-free bottom-fed RDA (meaning the 510 connector positive post is drilled to pass liquid). The drilled, widely spaced posts over a ceramic base make for a safer and easy to coil and wick setup. The best part of this system is after you squeeze (squonk) the bottle to flood the atty and coil/wick, the expanding bottle sucks any excess juice back down out of the atty. You need only enough wick to fill the coil.
A well-worn raw aluminum REO Grand is the only mod I take out of the house. With a 6 ml bottle and 18650 battery, in a package much smaller than a pack of cigarettes, out of sight in a back pocket, I'm set for a 15-16 hour day. The first thing I do when I get home is top off the tank with my stealthy work-day vape and swap in a fresh battery--it's always ready to go. An external tank would be redundant and unneeded. No spare juice or battery needed either. If I could have only one mod, it would be a REO Grand.
Now all this being said, I enjoy other gear. Since I've had it, my Super-T Simplicity 18500 has displaced my second REO Grand as my main at home mod, primarily for dripping, the act of which I enjoy immensely. I've also used tanked cartos on it for a couple of juices, at least until I got Sophia (which is compatible with the REO). I also have a large VV/VW mod that rarely gets any love. Mechs are my mainstays and the REO Grand is one of the best.