Various is the correct author for any book with multiple unknown authors, and is acceptable for books with multiple known authors, especially if not all are known or the list is very long (over 50).
If an editor is known, however, Various is not necessary. List the name of the editor as the primary author (with role "editor"). Contributing authors' names follow it.
Note: WorldCat is an excellent resource for finding author information and contents of anthologies.
This is a sampler for five (5) graphic novels by different authors:
Be Prepared by Vera Brosgol- The author's memoir about spending the summer at a Russian summer camp and going through drama and shenanigans.
All Summer Long by Hope Larson- A coming-of-age-story of a teen named Bina over the summer.
The Amazing Crafty Cat- A little girl named Birdie, when getting herself into a sticky situation, turns into her alter-ego, Crafty Cat, who can craft her way out of anything. (So cute, so pure)
Zita the Spacegirl- After pressing a button that came from a meteorite, Zita's friend disappears. It's up to Zita to embark on a wild adventure rescue her friend.
Real Friends by Shannon Hale- (The only one in the sampler I read the full version) A memoir about the author's memories and childhood experience with childhood.
The first 2 were pretty cute. I might actually look for the rest of them. The last ones were a bit more juvenile for me. I am obviously not the target audience, being in my 30s. I think all of these would be good for a middle grade or a little younger audience.