I don't really have a specific favourite, it's more of a favourite species, it's literally any symbiote. There's something I find really interesting about the symbiote, the way they can bring out the worst in any creature makes for really interesting stories.
Doc Ock. He was the main villain when I first started reading Spider-Man. He'd have a major appearance every 2-3 years and while some were better than others, it was always an important story before the era of event storytelling. One of the best stories was the Dr. Octopus/Owl war in Spectacular Spider-Man around 1983. He was well known to me through cartoons and also from Marvel Tales reprints, and some of my first back issue purchases. That Spectacular Spider-Man story, though, was a perfect culmination of all their past encounters. Afterwards, Doc Ock would have some uneventful appearances or stories that didn't live up to their potential, but he was always interesting.