Who are considered the most important salsa composers/performers?
Some of the best (for rhythm, lyrics, and/or popularity) are 'Fruko y Sus Tesos,' 'Grupo Niche,' Celia Cruz, Tito Puente, Hector Lavoe, Joe Arroyo, Willie Colon, 'La Sonora Matancera,' 'El Gran Combo de Puerto Rico,' and Ruben Blades.
Later addition: I should have remembered Oscar D'León. I can almost hear his song "Llorarás" in my mind, now that I was reminded of him by "zookid" (and the link to the 'cantautor''s Wikipedia article) in his answer. Somehow, though, his band name doesn't seem to me to be as famous as his own name.
Another addition: I recently remembered the "Fania All-Stars" (known to me usually as just "La Fania"). According to Wikipedia, it includes many of the performers mentioned above.