Top of the Line is a canceled episode. It had a script that, unfortunately, wasn't recovered from Viacom and Nickelodeon.
Details are unknown 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) , Eric Trueheart says that this episode would have included an appearance from Tak.
Little is known about the episode. What is known, however,  is that there would be a SIR competition (presumably an arena battle or a series of challenges), which GIR wins and in the process destroys all the other competing SIRs except two. 
The purpose of the competition is presumably to prove which Invader was superior to the others.