Computer-generated imagery

Computer-generated imagery (CGI) in Transformers media has gone through four phases thus far.

Generation 2
The Generation 2 "cartoon" was simply the Generation 1 cartoon with a new CGI opening, and new scene-transitions featuring the nefarious Cybernet Space Cube. These added nothing but glitz to the cartoon, and are best forgotten.

The Beast Era
Both Beast Wars and Beast Machines were fully CGI, created by Mainframe Entertainment of Canada. Beast Wars was a bit primitive, even in its day. (It is notorious for lacking shadows except when vital.) Beast Machines, by contrast, holds up reasonably well even by modern standards. Both are praised for their good use of facial expressions and body language.

Energon and Cybertron
The last two-thirds of the Unicron Trilogy, Energon and Cybertron, both used shaded CGI for the Transformer characters, and traditional cel animation for almost everything else (humans, backgrounds, etc.). Some episodes (e.g., "City") used CGI for doing complex environments, particularly when the camera needed to be able to move through the city quickly.

In comparison to the Beast Era, the character animation in Energon and Cybertron is very unimpressive. For an extensive discussion of its flaws, see Energon, Production flaws.

On the positive side, the character models are toy-accurate to a fault, reproducing the design and transformations of the toys in detail.

Transformers (2007)
The live action Transformers movie franchise, obviously, uses CGI extensively, most spectacularly for almost all appearances of the Transformer characters in robot mode.

Trivia

 * If one includes the Generation 2 cartoon, then of the nine Transformers TV shows that have aired in the U.S., more than half have used CGI, and nearly half have used it extensively.