Baby Tic Tac Toe was my childhood one and I also think she gets forgotten a fair bit by the fact that she's shunted out of the only TV ep I had starring them as a kid - Ice Cream Wars. She also didn't get as much attention in the comics as some of the others did. I always feel like she needed more attention.
Over here because we didn't have mother ponies for Bouncy, Fifi or Quackers, it helped make them ponies in their own right so none of them seem neglected to me. I can see the argument for baby Lickety but since the UK comic took the time to introduce her specifically as the cousin of the Play and Care baby Lickety Split (something it never did for alt versions of any other character) and she starred a lot in the anime, I can't really see it.
I'm going with Baby Tic Tac Toe.