I haven't researched hugely
But...
i think in terms of rock music.
Emerson
Wakeman
Lord
Hopkins
stand out quite alot.
note this is for rock songs, involving keyboard/or piano/organ in Hopkins or Lord and a few of the other pieces.
my 2c
btw, is it just me, but are most of those songs they play not greatly challenging - its mostly timing and sliding up the keyboard... i.e. the 'good' Bach/Beethoven or Joplin/Lanz stuff is much harder