I don't think you really want the most actively developed branch, but rather the most stable branch. My Arcueid is very actively developed, but I would definitely not recommend you use it for a production site!
Unless Mr. Graham is presently using a new version of Arc that isn't publicly available to run this forum and Hacker News, the fact that these sites are running well should be a good sign. I think that unless there are extensions you need that the reference Arc 3.1 does not provide, you can't go very wrong there. I've been using the Anarki (nex3) branch as well, and it is fine.
