Growing up, my Nana and Papa had a pear tree, and Nana used her apple pie recipe but with those pears instead. She only did it once as far as I know, and later they cut down the tree, but I still miss that pie!
Overall, probably strawberry rhubarb, despite the fact that I'm otherwise not a big strawberry fan.
However, I make lemon meringue pie from scratch every now and then, and my lemon meringue pie is my favorite pie of all. I've gotten my crust recipe dialed in, and I've gotten my curd making tuned for exactly the right consistency. After I got mine down, I don't like other people's as much as I used to.
But key lime pie? It isn't my favorite, but damn is it good :)