I've seen some recipes, like this one, but they don't specify which type of onion to use.
What kind of onion is good for onion rings? E.g. is it better to use yellow or red, sweet or tart etc?
You can use any onion variety you want; it depends on your tastes (and availability). White onions have a shaper, more pungent flavor, while yellow onions have a nice balance of sharp and sweet (and become sweeter the longer they're cooked). Sweet onions (Vidalia, Texas, Maui, Walla Walla) are not as astringent and have a softer and, well, sweet taste. As for reds, they're similar to yellow flavor-wise, but slightly milder.