I have to agree that the Nezumi win this contest for me because they have a culture that doesn't revolve around being a campaign villain, and even with their social drawbacks, they can still be part of an adventuring party in OA/Rokugan. I personally use a homebrew version of "Nezumi" for my own games, balanced for use with the Arcana Unearthed/Evolved rules set, and using the conceits of that system (3 racial levels, sneak instead of hide and move silently, etc.)
Robert "Born in the Year of the Rat" Ranting