Peanut butter and I am glad I introduced it early. It was adviced by our doctor to do so to prevent peanut allergies. I wish I had also given something like almond butter to prevent tree nut allergies as we now have a tree nut allergy for our middle child.
Alas, we introduced peanut butter early with our third child, also at our doctor's recommendation, and sadly he is the only one of the three with a (severe) peanut allergy. It's so hard to know how to handle allergenic foods when the advice seems to change every few years!