OK, so i jest about the Pet Shop Boys and The Spice Girls.

This is one of those unanswerable questions. Its a bit like the Red Triumphs are better then the Black ones are better than the Blue ones are better then the Pink ones.
Genesis, Brilliant, but how can you compare them to the likes of Led Zep or Deep Purple. Same goes for Pink Floyd. Rock music covers so many types of music it is very hard to pick a "Best Rock Group".
So many great bands have been mentioned here, but so to have many been left off.
What about Black Sabbath, Rainbow, Judus Priest, Whitesnake, UFO, Sorpions, Status Quo. There are so many brilliant bands.
And what do you use to gauge the Best Rock Band. Albums Sold, number of paying fans at concerts, or length of time the band has been playing. Use those guidelines and KISS are up there with the best of them, and ahead of most.
Or, do you use musical ability. If so, well that knocks out the likes of KISS, AC/DC and a million over heavey rock/metal bands. Not exactly at the forfront of musical abillity are they.
Who is the best Rock Band of all time is the all time unanswerable question and will be argued for the rest of time.