I agree that it depends on what I'm making. I try to keep several varieties of onion (along with shallots) in my pantry at all times.
Regular yellow onions are used most. Sweet onions like Vidalias are used when onions predominate in a recipe, since husband isn't a strong onion fan. I also enjoy them raw on tacos, enchiladas, burgers, etc. Same goes for red onions - great raw on burgers or sauteed into my Kale/Feta/Pasta dish. Shallots I use all over the place.
So most of the time I have a sack of regular yellow onions, some Vidalias, some reds, shallots, scallions (& tons of garlic) on hand at all times. Can you tell that I love alliums?