Everything from Megatron's voice to his personality makes him a memorable villain from the 80's. His layered plots against the Autobots always had me going "
How the hell are the Autobots going to be able to defeat The Decepticons today?!?!?" It always seemed like insurmountable odds. Looking back, Megatron's army was pretty much on point as he had Starscream (most of the time), Soundwave, Shockwave, Devestator, etc. They really were the better army. I thought Galvatron's first appearance was great, but after that he became a rather comical character. On another note: Megatron was voiced by Frank Welker, who is like the second GOAT voice actor next to Mel Blanc.
I would also say that Starscream was pretty memorable villain also. When Wizard Magazine poster their list of the Greatest Villains of All Time, Starscream was actually ranked ahead of Megatron due to his treacherous nature.