Someone who can provide reasonable assurances that give the recruiter a warm fuzzy that you were not up to no good.
It doesn't matter if you were sitting on the couch playing video games or doing volunteer work building orphanages...the important thing is to have someone who can vouch for you, preferably not family (or slacker/gamer/stoner buddies).
A pastor, doctor, coordinator of volunteer work, school official, even a teacher is what you need.
This is not about what you were doing, it's about what you weren't doing...you want to make it very easy for them to put that question to rest. If you don't have a great reference, it may not be the end of the world, but you might get put into the stack of resumes which need extra research...recruiters are very busy, they'll probably never get around to that stack unless you have inside connections.