Top of the Line is a canceled episode. It had a script that, unfortunately, wasn't recovered from Viacom and Nickelodeon and it’s currently unknown if it’s still saved somewhere or lost after the series untimely cancellation.
Details are a mystery since neither the script or audio were ever finished and/or released. In the Fan Q&A section of The Medium-Sized Book of Zim Scripts: Vol 1: Pigs 'n' Waffles, Eric Trueheart says that this episode would have included an appearance from Tak .
Little is known about the episode. What is known, however, [citation needed] is that there would be a SIR competition (assumed to be an arena battle or a series of challenges), and the competition presumably would have taken place in a planet Conquered by the Irkens rather than Irk itself. GIR would ultimately be the winner and in the process destroyed all the other competing SIRs except for Invader Skoodge’s and Invader Tenn’s SIR Units. [citation needed]
The purpose of the competition is presumably to prove which Invader was superior to the others. [citation needed]