Top of the Line is a canceled episode. It had a script that, unfortunately, wasn't recovered from Viacom/Nickelodeon.
Little is known about the episode. What is known, however, is that Tak was to be the main antagonist, and 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.
Another possibility is to decide the best SIR unit of them all (hence the title). Invader Tenn would have also made an appearance and had a somewhat significant role, or, at least, more than she had in previous episodes. Skoodge would have helped Zim enter the tournament, but would have had his robot eliminated, quite possibly due to Tak's sabotaging.