I've cast my vote for Rainbow. For the most part, I've only ever used Arc as a topic for discussion, as shallow inspiration (e.g. naming conventions), or as a platform for language-implementation-independent libraries I rarely used. But there have been two exceptions:
I briefly maintained my website's static site generator as an Arc+Penknife project. It could run on Arc 3.1, Anarki, and Rainbow, but I preferred to run it using Rainbow for speed.
At one point I wanted to fill a gap in our Arc-in-the-browser discussions, and I especially wanted to show there was no innate reason for ClojureScript to omit 'eval, so I spent a lot of effort porting and adapting Rainbow to make Rainbow.js. I figure this means I have a special investment in Rainbow, even if I don't run Rainbow or Rainbow.js for any serious business.
---
"Do people think I should add even deeper spin-offs like nulan and lathe and blade?"
Lathe and Blade shouldn't make it on this list.
Lathe is a suite of language-implementation-independent foundational Arc libraries, some of which specifically make it easier to write language-implementation-independent Arc code. I suppose you could program for Lathe as though it's an Arc platform, but that just means polyglot programming for several Arc platforms all at once.
Blade was a language project that never got off the ground, and it pretty much didn't have anything to do with Arc.
