I guess that the most common filling for pumpkin pies in the US is Small Sugar (aka New England Pie) pumpkin. It is a Cucurbita pepo, so it is in the same species as Jack o'Lantern pumpkins and some others.
Another common choice according to what I've read is the Cushaw squash. That one is a Cucurbita mixta and not so closely related to the ones we usually think of as "pumpkins."
Except for when I've bought cans of pumpkin, these have never been in my pumpkin pies. Making these pies goes back to when I was a kid and would be sent out to get a Pink Banana Squash. I'd have to break that squash open with an axe on the chopping block .
Pink Banana is a Cucurbita maxima and since Burgess Buttercup does well in my garden where I live these days, I am continuing to use a C. maxima in my pumpkin pies. How about you? Do you have a traditional or not-so-traditional choice for pumpkin pies?