The Beatles released a lot of fantastic music, and you'll find more classic songs on an album like Revolver than in most bands' entire discography. While they're not my personal favourite band, I've got no problem with those who claim that they're the greatest band ever, since by most standards of measurement they just are. They've sold more albums than anyone else, had more number one singles than anyone else, and had a huge impact, culturally and musically. That's about as close as you're ever going to get to definitively calling any group the "best ever".