To me, Led Zeppelin and Queen are one of the greatest and definitely one of the most influential bands of all time. I don't just say that because I grew up listening to them. I think there are many, many artists and bands out there that wouldn't be around had they not been inspired by them. For Zeppelin, they had such a unique sound and transcended so many genre's, and the originality and creativity those guys have is unbelievable. You can argue that no band really needs a twenty minute drum solo, and I see where people are coming from when they say that, but to me it's not really overzealous. Just the simple fact that
anybody can go for that long and make it sound as good as John Bonham did just blows my mind. And Jimmy Page's guitar skills?
And Robert Plant's voice? Don't even get me started. Nowadays there are so many tricks you can use in the studio to make your voice sound perfect, stuff that probably didn't even exist in the 60's and 70's, and yet his voice always sounded so perfect and natural, not only on their albums, but live, too. I nearly got teary-eyed while watching The Song Remains the Same one time because his voice just sounds so gorgeous. He's my favorite singer in the world.
Same exact thing can be said for Queen. There's a very good reason why so many people regard Freddie Mercury as a legend. He was so different, probably considered really odd by some people, but he was so proud and never held anything back. Again, I think one of the most influential singers in music. He was so charismatic, but obviously it wasn't just him that makes Queen amazing. All of them are incredibly unique, something that's definitely lacking in the music business nowadays. Originality is just so important in music, to me at least. I don't think there's that many people who dislike them. (I know there are certainly people that don't, but I can't say I've ever met one.
)
"Greatest" is such a subjective term and you can argue for hours what makes an artist "the greatest," but I know those two are the greatest to me. That's all just my opinion.