I meant Vidalia
In my opinion, any sweet onion will do. The key to success is carmelizing the onions well. This is my current favorite onion soup http://www.food52.com/recipes...
Well, don't use red, for starters. If you use a sweet onion you'll cheat yourself out of watching an ordinary yellow onion transform from a thing that would like to burn your eyes away into a near candy. Add stock, reduce. Add stock, reduce. An enchanting process.
I'd use yellow onions, too. As others have said, they'll provide all the sweetness you need, if cooked long enough and with sufficient care. ;o)
Love to throw Alton Brown's name around and I have used his advice and find a red spanish onion to be wonderful Alton says: "5 sweet onions (like Vidalias) or a combination of sweet and red onions"
I've always used a sweet onion... Vidella, Bermuda, walla walla, etc. I don't add the stock until the onions are done to my preference. A sautéed onion is better than boiled.
I'm going to dis Alton Brown (who looks like a celery root) here . Definitely use a sweet onion, Vidalia et al. The two most important things in regard to onion soup are 1) the onions have to slowly caramelize for about 45 minutes and 2) you need to use really good beef stock made from bones rich in marrow. Also, when it's done don't throw a giant glop of cheese on top---that is so 70's. The classic is a crouton with melted gruyere. Trust me, I've done this hundreds of times.
I usually use plain old yellow onion or sometimes Spanish onions. As long as you give them enough time to slowly caramelize, it should be good to go.
I find Vidalias too sweet in FOS. Especially if you use a long oven caramelize method. Like in Cooks Illustrated FOS recipe. In Feburary, the yellow onions here are pretty high sulfur and bitter from being stored so long, and they benifit from the oven method.
So, I use both--depending on the time of the year and the harshness of Yellow Onions, and availability of Vadalias. When onions get bitter and harsh here's the method to use: http://www.food.com/recipe...
But don't use that with sweet onions...it makes the soup far too sweet. It's my fall back method for when we have harsh onions at the store.