The J.Crew Jackie cardigan. I've been buying one or two or three a year for at least 10 years. But jean/tank/cardigan is pretty much my go-to outfit, so they get a lot of use.
I'll have to this out---I substitute teach and my uniform is cardigan, tshirt, and slacks. I have about 15 Lands End cardigans---love that if little people give me peanut butter sandwich hugs, I can just throw them in the wash!!!