The Voron 2.4 is a superb design and was the basis for my Ft5 to v2.4 "Boron" conversion. At a minimum layer height of 0.025mm, I really think that's the design to beat. The quad gantry might not have the cool planar articulation of the Rat Rig, but I think the fidelity of the Voron is an acceptable tradeoff, among other neat features.
Another printer that I'd mention that's missing here is the Voron v0. Now, before I come out sounding like a fanboy, hear me out: The v0 takes ~30 seconds to heat everything up and start printing! For rapid prototyping (120mm^3), this thing is a beast. I can iterate, and it'll have a part cranked out by the time I've got the next design ready. If you already have a ~300mm form factor, take a look at the v0. The small form factor and 'cute' 1515 extrusions are almost too much to pass up on that alone. You owe it to yourself to check this thing out!
I'll also echo Ron's sentiment about the v1.8. I have a custom job that runs similar to this design. If I could imagine an FT5-r2, this would be it. Not far off from what @csorrows drafted. (Love the design!).