Yeah I get that, but I feel that saying who's best is difficult in itself, but like my first post indicates Black Sabbath have one hell of a claim to the title!
Eh, I think they have too much stacked against them honestly. While the Ozzie albums have a certain charm in the vocals, his voice is really terrible. I also think a lot of the style has aged terribly. Not so much because it's bad in any way, but because of how often it's been used and ultimately shit on by probably hundreds of terrible bands. But the guitar play and lyrics are still amazing. I love me some Black Sabbath though.
And like you said, it's hard to really say best band because of taste and so many other factors but here's my reasoning for CCR.
Susie Q
I Put A Spell On You
Bad Moon Rising
Down On The Corner
Fortunate Son
Who'll Stop The Rain
Lookin Out My Back Door
Have You Ever Seen The Rain
That's just a small sample of their discography but look what it entails. They've mastered the hippie rock, hypnotic psychadelic rock, Southern Rock, protest rock and even have bonafide classics that transcend musical genres that everyone knows. If not the greatest rock band, definitely the greatest american rock band IMO.