So far the countdown has been:
10 - Bumblebee
9 - Whirl
8 - Mattel/IGA
7 - Age of Extinction
6 - Spotlight: Kup
5 - Exhaust
4. Unicron
Unicron has been used sparingly since Floro Dery designed him for the 1986 movie. In 20 years Unicron's appearances have been few and far between as it is hard to write unique stories around a character who devours planets.
It was Simon Furman who penned Unicron's backstory of a Dark God locked in eternal battle with Primus which has been tweaked a little over the years. By current lore Unicron can change between realities and has devoured 20% of all known universes.
Let's process that for a minute - Unicron has single-handedly killed a staggering number of beings both robotic and organic and everything in between. Megatron is credited with the deaths of billions but Unicron's bodycount is easily in the centillions (a number followed by 30 digits) if not higher, making him one of the most prolific killers in possibly any fiction alongside Thanos and The Anti Monitor.
Here's some perspective:
Deaths and person responsible
33 - Freddie Krueger (Nightmare on Elm Street Movies)
73 - Detective John McClane (Die Hard movies)
118 - John Preston (Equilibrium Movie)
220 - John Rambo (First Blood/Rambo movies)
488 - The Joker (DC comics)
1,500+ - The Punisher (Marvel comics)
Here's some perspective:
Deaths and person responsible
33 - Freddie Krueger (Nightmare on Elm Street Movies)
73 - Detective John McClane (Die Hard movies)
118 - John Preston (Equilibrium Movie)
220 - John Rambo (First Blood/Rambo movies)
488 - The Joker (DC comics)
1,500+ - The Punisher (Marvel comics)
2,000,000+ - Ultron (Marvel comics)
2,300,000+ - Luke Skywalker (Star Wars: A New Hope)
2,000,000,000+ - Grand Moff Tarkin (Star Wars: A New Hope)
3,000,000,000+ - Skynet (Terminator movies)
6,000,000,000+ - Nero (Star Trek reboot movie)
61,000,000,000 - Paul Atreides (Dune)
10,000,000,000,000,000,000,000,000,000,000+ - Unicron
That's a lot of zeroes and yet he isn't the most horrific thing in the Transformers universe...