The various alternate realities in which the Transformers franchise take place.
The G1 reality is considered the primary reality of the Transformers franchise as it was the reality that the original anime, Japanese manga, US comics, and many of their spin-offs took place. This reality features the bulk of the franchise as seen in the 1980's and 1990's and is still ongoing in the IDW comic books.
Major G1 series
The Transformers The Movie
Beast Wars Transformers
Beast Machines Transformers
Transformers Robots in Disguise (the main villain Megatron 2 is from the G1 reality)
As the canon storylines of the US and Japanese G1 stories differ, the Japanese reality is considered to be a separate reality from US continuity.
Major Japanese G1 series
Transformers G1 (without the return of Optimus Prime trilogy)
Transformers The Headmasters
Beast Wars Transformers
Beast Wars II
Beast Wars Neo
Transformers Robots in Disguise (again due to Megatron 2)
Henkai! Henkai! Transformers
Generation 2 is a sequel to Generation 1. it is in a sense the same as G1, though the US and Japanese versions differ greatly. A common aspect was Optimus Prime and Megatron as uneasy allies. G2 stories were told in a Marvel Comics series and a Kodansha manga. The anime Transformers Generation 2 was actually just a rehash of G1 episodes, with the Cosmic Cube and some new CGI cutscenes thrown in.
The reality Transformers Armada/Energon/Cybertron and their comics and manga spin-offs take place
The reality the 4 live action Transformers movies take place within.
The reality Transformers Animated takes place within, this includes the anime and US comics and Japanese manga related to it. The animated reality has even been given it's own version of the Shattered Glass reality.
The reality that Transformers Prime, Transformers Rescue Bots, many of the IDW comics, and the video games Transformers The War For Cybertron, and Transformers The Fall of Cybertron take place within.
Stand-alone realities incidental realities
The Brazilian G1 incorporates all of US G1 and Japanese G1 anime, the anime was considered a bootleg though
The Gobots reality has recently after Hasbro and Takara obtained the rights to the Gobots been added into the Transformers multiverse.
The comic book reality where the Autobots are evil and the Decepticons are good.
Robots in Disguise
Though Megatron 2 is from the G1 reality, the rest of the characters are from a stand-alone universe in this series
All Autobots, Maximals, Decepticons, and Predacons seen in Robot Masters are from different universes.