If you think Sierra Nevada makes the best brew, I have a hard time believing you have experienced enough different brews to form an opinion. There are thousands of brewery's out there. Sierra Nevada is a decent brew but to call them (or Fat Tire for that matter) the best is the equivalent to saying Lil' Wayne is the best rapper alive. The only people who say that are one's who don't know what they are talking about.
Here are a couple that everyone should check out......
http://www.alesmith.com/
AleSmith is my personal favorite. IMO they make the best IPA. It is hard to find at this point unless you are in San Diego. They are however making a name for themselves and I'm sure their brews will become more readily available.
http://www.stonebrew.com/
Stone is a lot easier to find then AleSmith. Their Arrogant Bastard Ale is some of the best brew out there.
And here are a couple good sites to read about and discover new brewing companies.
http://beeradvocate.com/
http://www.ratebeer.com/