What is "plain" ice cream?

I'd say Vanilla since, generally speaking, more plain ice cream isn't served. It's considered plain because it's white and pure.

Vanilla has even evolved into a term for something's original. Example: There is a fairly popular mod for Super Smash Brothers Brawl called Brawl+. So people who deal with the two terms call Brawl+... "Brawl Plus" and the original Brawl, "Vanilla Brawl."