Prime is probably my fourth favourite.

G1, Beast MACHINES, Beast Wars, Prime, Cybertron/Armada no real other ranking.

I really loved Beast Machines and Beast Wars because of how much it expanded on the original G1 cartoon universe, and although still some questions I had weren't answered, like, what happened after Galvatron and Skorponok plus what was left of the Decepticons left Cybertron, what happened to the Autobots after the Golden Age? I know they eventionally became Maximals and Predacons, but when and how did this occur. (Forgive me if some of these were actually answered, I haven't watched em in a while haha.)

I especially loved the Ghost Starscream episode because it was one of the earliest tie ins to G1.