It's Planet Namek, because it's the most well constructed story arc. The Saiyan Saga is good, but basically a prelude to the Namek Saga. The Android/Cell Saga suffers a bit from this clown car effect where Toriyama was forced to keep introducing new villains until his editor found one he liked. The Buu Saga has some strong moments, but much of it is incoherent and random.