I am a guitarist and I don't get what people mean by saying they don't like "the" keyboard sound of Keith Emerson. As KevShmev said, he used a ton of different sounds, most of them pretty traditional (organ, piano), especially on the first 5 albums, combined with some experimental sounds, some of which he was the pioneer of.
I have to say, and I'm sure, Jordan Rudess would agree with me if he could read this, Keith Emerson was one of the greatest keyboard player the world has ever seen, definitely in the top 3. I met Jordan on a meet & greet two days after Keith died, he wasn't quite himself. He was all mellow and looking on the ground not saying much. I could imagine this being because one of his idols died.
Greg Lake certainly had one of the greatest voices in Prog history. Just listen to Karn Evil 9, there's barely a melody to sing, and yet he makes it sound great. Also he was an underrated bass player. Pictures At An Exhibition never fails to amaze me. Three people (3!!!) on stage creating THAT wall of sound. Wow.