Slackware. The thinking man's distribution. Trim it down ultralight purpose built, or keep it ready to go at a moment's notice. I've got Raspbian on mine, but once I get the 8 gig version, I think I may just pull the trigger on Slacking it up.

Then again, I may be completely insane, because I'm humming and hawing around building a Single System Image out of a pair of them and actually making something roughly analogous to a laptop to go along with it.

To be frank, you probably shouldn't take my advice. I'm clearly a glutton for punishment, and it may lead you to the darker corners of computing related insanity.