What kind if onion do you use when making chicken stock?

Just yellow onion-not peeled... gives golden hue to broth. No leek-funky smell and ...just no.

I use yellow onions, Spanish onions.

I'm dreading the next couple of months tho. For some reason, onions in Feb, March..are very high sulfur. Red onions which can normally be used raw, are bitter. Even the yellow onions seem to acquire a bitter taste.
And "sweet onions" are going soft at this time.

In my stock, I use yellow onion + green tops of leeks that I've saved and frozen for this purpose. Plus a bit of carrot and a big handful of parsley stems (full of flavor, also saved for this purpose.) Have some bubbling on the stove now, in fact. ;o)

I like yellow onion with the addition of 50% leek (1 c onion with 1/5 c leek). I find this gives the best flavor in anything, soup, stock. anything

I always start with a lightly sauteed classic mirepoix, carrot, celery, sweet or yellow onion (1:1:2 ratio), leek is used in place of onion for a milder flavor, bouquet garni includes thyme, parsley stems, bay leaf, a few peppercorns. Jmburns is right, purple/red onions will turn your stock a horrible color!

Onion is not exactly, absolutely necessary in chicken stock but I would use either a brown onion or a sweet onion---vidalia, walla walla etc. More important are the other aromatic flavors, in particular your bouqet garnie (celery tied up with parsley and bay leaves and maybe time). Also you can substitute leek leaves for the celery in the garnie bundle and not worry about the onion at all.

I just use a Spanish onion. No red or it will discolor stock. Sweet onions may alter the flavor of the stock.