I'd say King Crimson, although they're not my favorite band or any of their albums my favorite album.
But they were argueably one of the earliest Prog bands, In the court was revolutionary in 1969. They've constantly pushed the envelope of prog, Lizard in 1973, Larks Tongue in 1973, Red in 1974, Discipline in 1980 - all are wildly different from each other and unique for their time.

Genesis, Yes, ELP, Kansas, Rush, Genital Giant, Van der Graff, Pink Floyd, all are terrific and put out some great albums, but not they all stayed within a narrower range, they didn't stray too far from their signiture sound. Crim was the most progressive, always searching, exploring new ground.
That's the measure of a best progressive band for me.

I have Korai Orom's 1984 album, it's good, hypnotic like a more musically achomplished Hawkwind, but they don't make the top ten for me. Hey, I've got over 1500 CD's so I get to be pickey. Just my .02