Favorite is signed by tapping your middle finger on your chin. You take an open palm, indent your middle finger slightly, and tap it on your chin.
The same sign is used for taste (sometimes with the finger tapped a little higher on the lips instead of on the chin).
Favorite is a useful concept with older babies, because it introduces the concept of choice and making decisions. For snacks, you can give your baby a couple of different options and ask, "Which one is your favorite?" When they respond, you can ask them to append favorite to their answer. For example, signing back, "Cracker (is my) favorite," instead of just, "Cracker."
As parents, it is our job to introduce our children to a variety of foods that enhance their health, nutrition, and appreciation for diversity. The sign for taste or favorite can be introduced in tandem with new food items.