Bourbon and Branch is a well-hidden gem in the heart of the Tenderloin. It's so well-hidden, in fact, that you have to visit their website to get full disclosure about the location of this place.
This authentic speakeasy boasts an extensive menu (of biblical proportions) that features specialty, top-top shelf drinks. If you have a group, it's best to make a reservation--you'll be given a secret password to enter--and keep it under six people, as the cozy booths only seat three on each side. If you're feeling more spontaneous, The Library on the back side of the building, is open to the public without a reservation. They also boast some impressive cocktails, but the menu on this side is significantly smaller.
Drinks range between $11 and $20 dollars. It's best to make a 2-hour reservation block with plans to drink one cocktail per hour. The wait staff is helpful and friendly, but expect there to be a significant lag time for drinks, as mixologists behind the bar take pride in their work.
And don't plan on using your mobile phone in this joint--it's against the rules; you might want to read up on these before you go in.