Oh that’s a tough one, but I’d have to say it’s a tie between the arch where Megatron + the gang go to the alternate universe Lost Light to get Rewind back AND the first Overlord arch when Rewind dies. I would say the Overlord arch as #1 if it hadn’t been for the fact that seeing the entire Lost Light crew absolutely decimated by the DJD (who’s intro panel was Excellent btw) was 100x more horrific than Sunder ever was to me, not to mention we get Rewind back with the iconic moment when he is reunited with Chromedome. 

However, it would be a damn crime to try and push off the Overlord arch, since we not only get a look into his psyche as well as the culture of the Cybertronian underground, but also Rewind’s death, Chromedome’s reaction, and Rewind’s Goodbye kick my ass in an entirely different way than the previous arch does. Those two issues were the ones that brought me the most emotions, but in different ways, so I can’t try to dismiss one over the other.

Honorable mentions would include: “Goodbye, little one”, the arch that deals with Krok’s trauma and selling Grimlock, and of course the one where Rung gets shot in the head for no good reason.