Cell saga, for me. Well, android saga, to be precise.
Toriyama is a good egg, and he deserves all the reputation he's earned over the years. However, I'd never been fond of how black and white his world was. All the characters were either purely good or laughably evil, and the only characters allowed to have an arc of any sort (like Piccolo or to a lesser extent Tien) where they go from evil to good tended to fall out of focus shortly afterward.
Then the androids hit the scene, being all chaotic neutral. I loved that entire arc so much that, to this day, 16,17, and 18 are my favorite characters in the entire show. They were just so different from everything else in the DB world at the time.
removing the androids from the equation, though, I'd still pick Cell's saga. I like Freeza more as a character, but his arc itself was a bit overly long. Cell's saga was better paced, and I enjoyed the light horror elements of its first half.