Generally when an old rock fan thinks of two great rock keyboard players, invariably two will consistently show up on the lists. Rick Wakeman of Yes is one that will get mentioned.
Wakeman was ahead of his time. However once he was convinced he was the star of the band, he had to buy new hats. His ego caused to leave the band for a while, for a solo career that didn't do so well. He finally made it back, but somewhat humbled. But all of this does not take away from his talent, one iota. Here is a live version of their first and best known hit, Roundabout:
The second keyboardist that will pop up, most assuredly would be Keith Emerson, of ELP. Keith's style was a little more cosmopolitan and jazzy, than Rick's. Rick was more classical. But both captured an art form in rock music. Whereas rock was primarily a blues-based genre with the central instrument being the guitar, both Yes and ELP took a calculated risk and fused a unique style with the keys prominent, or more prominent than the strings. It was like having two lead guitarists, only one was playing the keyboards.
Here is KarnEvil #9 - Live: