"Devastator says: your end is near!"
— City of Steel
Devastator was the first Transformers gestalt (if you don't count Reflector, and nobody does), a team of five or six robots who could not only transform into vehicles but also combine, Voltron-like, into a larger robot. Making his debut in the season 1 finale Heavy Metal War, Devastator quickly showed the power of the gestalt teams by being nearly invincible and tilting the balance of power back to the Decepticons, which the Autobots had enjoyed since the Dinobots were added to their team.
Unlike the gestalts who followed him, Devastator was not a Scramble City design — that is, he was not one large robot that formed the torso and four fully interchangeable robots that formed any arm or leg. The Constructicons could only combine one way (or maybe two, I forget if his left and right sides could be switched (actually they were switched fairly frequently in the cartoon, but that was animation errors)), and rather than having one larger robot that formed the torso, they used two regular-sized robots.
Despite being a fan favorite, however, Devastator was routinely shown to be out of his league when pitted against other gestalts. Before the Scramble City combiners were added to the show, he fought Omega Supreme a couple of times and was soundly defeated each time. When Starscream went rogue and created the Combaticons, they surprised Devastator by revealing their own ability to combine into Bruticus and handed Devastator his ass on a plate. In Scramble City, he was defeated by the Aerialbots without even bothering to form Superion. And in the opening miniseries to season 3, Five Faces of Darkness he was smashed apart into his component robots by Menasor. Twice. Devastator seemed to have trouble maintaining his combined form, in fact, having been previously blasted apart once by Optimus Prime and another time by Perceptor, each time with a well-placed shot to the chest plate.
In the absence of other gestalts or gestalt-sized robots such as Omega Supreme or Broadside, though, Devastator was nearly unstoppable. Like other gestalts, his brain was the combination of his component robots and thus mentally limited to what they could all agree on, but since his primary function was "crush enemies" this was rarely a problem. His big moment was in Transformers: The Movie, when he was the Decepticons' siege weapon against Autobot City, tearing apart fortifications with his bare hands and shrugging off point-blank shots from an artillery cannon. Later on, however, he was shaken apart into his component robots (again) by Rumble and Frenzy (Soundwave's cassettes take insults to their boss personally) when the Decepticons were fighting over leadership.
Despite all of this, his fan appeal is easy to see. Of all the gestalts, he is the most frequently referenced in fan art and is widely rumored to be in the upcoming sequel to the Michael Bay Transformers Movie (update: he was). As the first gestalt, he got the most exposure in the cartoon, and he was clearly the most unique looking due to his non-Scramble City design. Construction vehicles are just plain cool to begin with, and the wide variety of Constructicon vehicle designs stood in sharp contrast to the almost identical looking Aerialbots and Stunticons.
Devastator, you might not have been the most powerful gestalt, but you were the first, the most unique, and the best loved.