Kind of a loaded question there..
you're about to get 100 answers from the group... there are some good ones out there, the Thayer and Chandler Omni 4000, the Badger 100LG, the Iwata HP-CR and HP-CS to name a few, I would venture that the ones I mentioned are used by quite a few people here, if not a majority of the gravity fed users here on the forum.
Personally, I own the Iwata HP-CR and love it! I know many people that own the others though and have not heard a bad thing about any of them.