I've quite a few paintbrushes, because I work in three mediums -- watercolor, oils, acrylic -- and each has its own demands. This afternoon I took a snap of my favorite watercolor brushes.
From left to right:
FineTouch 712 Hake, size 2" (goat hair) -- The least expensive of my watercolor brushes, I use this for wetting large areas, and for dry texturing wood grain.
Winsor & Newton Series 140 Mop, size 7/8" (goat hair) -- A great brush for laying on washes and glazes.
Loew Cornell Arttec Bristle Supreme 500B Bright, size 1 (hog bristle) -- Invaluable scrub brush for lifting fine areas. I actually have two of these at the moment, as they wear down rather quickly.
Robert Simmons Sapphire S85 Series Rounds, sizes 12, 8, and 0. (red Kolinsky sable hair and synthetic filaments) -- The size 12 is my workhorse. I'd say that if I only had to have one brush, this would be the one, as it has the perfect balance of absorbancy and spring -- but is not as expensive as a brush made from pure sable.
American Painter 4050 Script, size 1 (Taklon) -- This is my go-to brush for making long, steady lines. The synthetic fibers keep it from splaying; a common fate for scripts/riggers.
Prang, size 7 ("natural hair"??) -- I've had this brush since about the 5th or 6th grade, as it came with my first watercolor set. Rather worn down, it's still good for making random foliage, and maintains a rather good nostalgia factor.
Testor's Shedproof Economy Flat Round, one size (nylon) -- After a lot of trial and error, I've found that this little number, meant for painting model airplanes and the like, is the only kind of brush that doesn't get ruined after a few uses by latex masking liquid. And best of all, they come in inexpensive packets of 2-3, so in the event that I DO gum things up, I always have a spare.