Tony Banks is easily my Number 1. He doesn't have the ridiculous technical chops of either Wakeman or Emerson, but he also never, ever overplays. He considers what's best for the song or instrumental 100% of the time, and has the best feel for overall composition and orchestration of the bunch. He's gone on to create soundtracks and two albums of great modern orchestral music using those skills, something none of the others has done.
Rick Wakeman and Keith Emerson constantly fight it out for Number 2 for me. Both have great chops, both in their prime (the 70's and somewhat into the 80's) had great musicality and a great choices in synth patches, but both have the tendency to overplay sometimes, putting the individual ahead of the ensemble. To be fair, that was part of the way their respective bands (ELP and Yes) worked. My second vote went to Emerson for having somewhat more restraint that Wakeman. Wakeman has a huge body of work (look up his discography some time) but a lot of it is complete shit, and he'll be the first to admit it. I haven't heard anything from him in over 10 years, and very little in the past 20, that has impressed me. Emerson at least knows when to stop.
Rick Wright is so often overlooked, so underrated, that I almost gave him a pity vote. I loved his work with Pink Floyd, and I enjoyed his solo albums as well. As a pure synthesist, he might be the best of the bunch. But he had a lack of technical chops, compositional skill, and overall body of work all working against him.