[Turbo_pro]

There is no correct answer.
It depends on the favor your attempting to produce.
I tend to use yellow onions when I'm cooking with heavier favors. Meats, pepper or tomato based sauces.
For lighter, more delicate affair, I use shallots or leeks. Some thing like Lobster in a champagne shallot sauce.
Leeks are especially good when you are heating for long periods like in soups or chowders where you're looking for a light favor but still maintain the structure of the onion.
When looking to add a sweeter favor, red onions are my preference. My best French Onion soup uses a mixer of Maui and red onions caramelized in port wine.