Jeff. Easy. Timmy was okay, and I never saw Cory. I don't know about Lassie as a solo act, not having seen him (her?) that way.
It's a preference that reflects my age (old enough to remember Jeff) as quality. I just couldn't warm up to Timmy even as the series itself was top quality. By the time Timmy left I was twelve and far less likely to watch an animal show like Lassie than a year or two earlier. There was a solo act dog show I loved around that same time: The Littlest Hobo, may be the best of its kind. Its stories, the human stories were highly engaging, dramatic, often bordered on tragedy. The dog was simply amazing.