+1 on buying yellow onions by the bagfull.
The yellows are less expensive and sweeter than most other onions. I like to caramelize a yellow onion or two with butter in a cast iron pan, and I use them in pastas, casseroles, chili, pretty much any savory dish.
I use the occasional red onion or shallot if a specific recipe calls for it, but when I make my own recipes, I always fall back on a good supply of yellow onions.
During the summer, I will sometimes use a larger yellow onion (or a big Vidalia or Walla Walla onion if they are available) on the grill when I'm cooking meat. Take off the outer skin, cut the onion in quarters form the top so you can open it up, put a chunk of butter and some seasoning salt in the middle of the onion, and wrap it all up in foil. Toss it on the grill, and it turns into a sweet buttery side dish for your cookout.