When one thinks of an organist, it's generally either their favorite blue hair from church or it's Jimmy Smith. I don't have enough comprehensive knowledge of the Hammond to know if he was the best, but he definitely was the icon for those into tone-heavy hard bop. If you're in that morbid "I shoulda bought more when he was alive, but I'll settle for buying some now" state, I recommend Root Down or a live Tokyo session called The Master (with Kenny Burrell). The latter has some terrific vocalizings underneath a few of the tracks that really add to the grit without losing the butter. The Cat and The Preacher are pretty good releases too.
If you have been thinking about how organ forms the foundation of what we call American Soul, I recommend checking out Jimmy McGriff with Les McCann, Ray Charles' Genius+Soul=Jazz, Dr. Lonnie Smith live anywhere you can (his studio albums don't do him justice and are generally turban-free), Booker T's work on Green Onions and backing up O. Redding and Bill Withers. If you wanna wander off into that nether region of Jamaican-American condiment Drew posted on earlier, check out the late Jackie Mittoo, a tremendous Reggae Hammond player from the 70s heyday. He does some straight-ahead soul organ work, along with some wonderfully dubby updates of 70s chestnuts like "Summer Breeze." Outsider jazz artist John Medeski typically does his best work on the Clavinet, but has worked with John Zorn on the Hammond in a few projects (including a terrific Electric Masada ensemble).