One that can hold an unwinding cake of yarn, of course!!
I've been knitting a lot with Toby in the stretchy wrap but the trailing ball of yarn was always a bit of an issue. How incredibly happy was I when I discovered that the hood of the ergo doubles as a very handy ball holder and the yarn unwinds very easily from it. Excellent! My knitting has just become even more portable.. hehehehe
As for the little baby, he has developed more than a passing fascination with the wool winder. If he's any where near it he tries to grab it and play with the handle crank, which is all very cute ... just as long as there's no yarn on it or I'm actually trying to wind a cake of yarn.