One of the main reasons why MMPR and the Zordon Era is so popular among Power Ranger fans is the characters of Bulk and Skull. While Turbo was airing, Fox Kids was experimenting with a spin-off Bulk and Skull TV series, but this project never left the writing board. Bulk and Skull were comic relief characters that developed from bullies to good nature bumblers and eventually to heroes. The humor involving these two was often campy and cheesy, but fans still loved it. Every year they were on the show there was a new theme for these two. For example, in MMPR S1 they were bullies, S2 they tried to reveal the identity of the Power Rangers, and S3 they became cops, and so on. However, being on the show for 6+ years, over time they would develop out of their one dimensional characteristics.
The two were actually written off at the end of Zeo when Skull's actor, Jason Narvy, wanted to leave the show. However, Bulk's actor Paul Schrier, convinced Narvy to stay on for two more years. Despite this, no adequate explanation was given to why them and Lieutenant Stone were back in the police force at the beginning of Turbo, especially given that they were fired from the force in Zeo and that Bulk and Skull would quit on Lieutenant Stone at the end of the season.
Despite their popularity for the comic relief they provided, some of the most popular Bulk and Skull moments are their most serious ones. These include the episodes When is a Ranger Not a Ranger from MMPR S2, Instrument of Destruction from Zeo, and Countdown to Destruction during In Space.
My personal favorite is Instrument of Destruction, when it revealed that Skull is a very good pianist. Bulk had dismissed classical music during the entire episode but came to terms with it and recognized Skull's musical talents. This episode did two things to develop these characters. Pretty much every episode before Instrument of Destruction portrayed Bulk and Skull as one-dimensional dummies with little or no talents. This episode however, showed that Skull was more than a one dimensional dummy, and did have some talent. The other thing it developed was how deep their bond was. Even though Bulk hated classical music, he cared more about Skull and came to recognize his talent.
Perhaps the most popular Bulk and Skull moment occurs simultaneously during Countdown to Destruction, which happens to be the most popular final battle ever in Power Rangers. After it appeared the rangers were defeated, Bulk and Skull reenacted the "I Am Spartacus" scene, each claiming to be a fellow power ranger. The citizens of Angel Grove would follow their lead. Later on after the Rangers morphed, Bulk and Skull led the charge to fight off the Quantitrons and to protect the rangers. This was a fitting end to their story, and for that matter, Countdown to Destruction would have been a fitting end to Power Rangers if it had ended there, which we know didn't happen.
Bulk showed up sparingly during Lost Galexy, while Skull made a cameo during the season premier. Both of them would make a cameo in the episode Forever Red, much to the delight of the PR faithful. At that time Bulk owned a cafe and called it "Bulkmeyers." Tommy happened to be there, and it seemed they knew his identity as a former power ranger by then.
Through it all, it's safe to say that Bulk and Skull are the most popular comic relief characters in Power Rangers. Dino Thunder tried to duplicate these two with Cassidy and Devin. Power Rangers Samurai will also try to parody these two, with Fat Jack and Spike. Though, it's unclear how it will turn out.