We all have them... Sometimes I'll pick up releases solely because they are from a favorite label, and always be satisfied by the result.
My favorite label hands down is Touch. Everything they put out is gold, and much of it is on a level of it's own. Lately I've been really drawn to their Touch Sevens line. The idea is to release 7 inch releases from various artists that capture the idea of a single piece of concise music. They have no indications of playback speed and contain locked grooves... I have all 8 of them, and they are a clear indication that the artists and creative heads at Touch are simply in love with music.
Runners up? I love 12k and Raster Noton. I also have a special place in my heart for both Paw Tracks and Kranky... And it's too bad that Erstwhile has all but dropped off the map in some ways.