There are many different lines of Transformer action figures, with various lines being produced only for Europe or Japan or America. Most of the lines are based on the various cartoon series or live action movies. Of course the oldest figures are generally the rarest and most collectable.
The first Generation from 1984 to 1990, also called Generation 1 or G1, these are THE Transformer action figures. The first cartoon series ran for five years in American from 1984 to 1988. Coinciding with the release of the cartoons on TV was the release of the action figures produced by Hasbro.
The Generation One toys include sub-line such as the Autobots (the heroes) and the Decepticons (the villains) . Of course they include the Optimus Prime and Megatron figures as well as lines for the Dinobots and Minispies. And include popular figures such as Starscream, Jetfire, Shockwave, Bumblebee etc. If you want to start a collection of Transformer action figures, this generation is the place to start.
Second Generation or Generation 2 or G2, was released from 1992 to 1995. A sad attempt to recapture the popularity of the G1 figures, but with no supporting series or movie, these figures failed to generate much interest or many sales.
Beast Wars made from 1996 to 2000. This set took the Transformer line in a very new direction. Instead of changing into cars or planes, these transformed into animals; Optimus Prime became Optimus Primal and changed into a huge Gorilla while Megatron changed into a Tyrannosaurus Rex. The Maximals were the heroes and the Predicons were the villains.
Machine Wars released in 1997 only released twelve figures in that one year, it was not very popular. It was sold exclusive by KB Toys.
Beast Machines released in 2000 and features good guys called Maximals and changed into animals were the bad guys were called Vehicons and changed into vehicles.
Robots in Disguise: Released in 2001, was a reboot of the original series, bringing back the old Autobot and Decepticon battle and these figures also supported a new cartoon series that ran for 39 episodes.
The Heroes of Cybertron released in 2002 the figures did not transform but had moveable arms and legs like traditional action figures.
Transformers: The Movie of 2007 and Transformers; Revenge of the Fallen released in 2009 revitalized the brand and of course several action figures were released to support the new movies. These figures are generally not as collectable as the older figures.