What is the best type of pumpkin to use? I am not familiar with pumpkins, but there are a ton of varieties and I imagine some are better for soup.

A pumpkin that tastes good! Hubbard squash by another name.

Delicata squash. For pies, "pumpkin" custard, or soup. Perfect roasted on its own too.

I use pie/sugar pumpkins. They are smaller and a bit darker in color. Very flavorful. DO NOT use the Jack o lantern variety. They are watery & tasteless. White pumpkins are edible, but your souper pie ends up being a strange wet cement color. Very un appealing.

The pumpkin family is a pretty large one- all members of the squash group. Here in America, we just tend to use the term pumpkin in reference to the traditional orange sort associated with jack-o-lanterns. For soup purposes, you can take your pick of any of the orange fleshed squashes, and not limit yourself to just the Halloween variety. If you do decide you want that kind, go for one that is either labeled a pie or sugar pumpkin. Avoid the monster jack-o-lantern sort, which tend to be watery, stringy, and flavorless. You can also use a butternut squash, turban's head, hubbard, or kabocha squash. Each brings a slightly different flavor to the party, some slightly sweeter or dryer, but all quite tasty. My family likes soup and pasta sauce, and filling made with all the sorts I mentioned. What I use often depends on what's available and looks best, sometimes what is on sale. For that matter, I have actually made "jack-o-lantern soup" too- to deal with an oopsy and sooth the tears of a youngster. It took a bit more seasoning, and less liquid, but in the end, it was still good.