If you're willing to put up with a big tank, I'll tell you what the absolute easiest RTA to work with is.
The Squape R.
It is hands-down the easiest RTA in the world to build on. It's almost fool proof. I've built on many RTAs and RBAs, and NOTHING is as easy as the Squape R to get right. The authentic is quite expensive, but EhPro makes a fabulous clone of it.
What makes the Squape R so easy? Quite simply, the swappable deck system. The deck in the Squape R has removable pieces that are tailor made for different build types. The decks fit over the posts and snap into place. Each deck has juice channels that double as a guide groove, so you just lay your screw/drill bit/whatever you wrap your coil around into the groove, make sure the coil is in the center, and then wrap the wires around the screws and tighten them. The deck is non-conductive so you don't have to worry about shorts if your wires touch it.
Then, when you wick the coil you simply run your wick through the same groove and trim it flush with the edge of the deck. Fluff the wick a little, then wet it down with juice, and that's it. On most RTAs you have to make sure you keep your wick outside of the juice channels and if you block them then you get dry hits and burn your wick. With the Squape R you put your wicks directly into the juice channels. It's just a lot easier.
The only thing you can really get wrong is the amount of wick you run through the coil. With cotton there is an easy method to wicking. Put the atomizer with the coil on your mod, stand your mod up, and run the wick through the coil. Then tug on one end of the wick a little. The wick should be snug in the coil, but not so snug that you start dragging the mod around. The wick should pull through the coil rather than pulling the atomizer.
There are 5 separate decks, but for subohm building you're really only going to consider 3 of them.
The W Deck is the most commonly used. It has wide juice channels for thick juices and lots of wick.
The D deck is for dual coil builds. It has two airflow holes, one under each coil, and each coil has it's own juice channels as well.
The B deck works like an RBA deck. It has a well that can be filled with wicking material which sits in a pool of juice, just like an RBA. The B deck still has the guide grooves to help you mount your coil though.
Here's a few picks to show how easy this RTA is to build on and the 3 decks I just listed.
Laying the coil in place... (This is on the S deck, so it's a small coil on a toothpick. The W deck is much larger and a 3mm drill bit fits easily into the groove)
W deck coil and wicking...
The B and D decks...