For me it’s Guardians. Those are just the BEST characters, it’s well acted, you could spend ages with them. Plus it has a certain je ne sais quoi that other superhero movies don’t have. I believe that phrase is French for “Kurt fucking Russell.”
As I said in another thread, however, all of these movies were of roughly similar quality and all pretty good. I’d rank Justice League second, Thor third. Lego Batman last.
I do agree that Captain Underpants was stellar, if it counts.