My favorite is a wrong back pair. This is a 1990 Score Wrong back pair of my 2 all time favorite players. This is an only known error card pair I got directly from Score back in the day. They have been my favorite cards from the second I got them till now. I can't imagine any card that could replace them as my favorite to own in my collection.
Besides being my favorite players they are on a red bordered card. I love red bordered cards and they look spectacular in the SGC slabs. I also have been collecting error cards since I started collecting. Then add in they are actually centered front and back when most wrong backs have a way off center back. This is one of those printing errors that turned out perfectly for me.
I am sure if I actually put them up for auction they wouldn't bring much. What I can guarantee is you could buy a nice condo on the ocean for less than I would sell them for.
So lets see some cards with stories on why they are your favorite.