For me it was The Magic Stealer! and The Power of... Point Dread! more than any of the other minicomics that shaped Eternia. I had a couple of other Series Two minis (where I met Ram Man and Man-E-Faces) but those stories were not quite as dramatic.
I had a couple of the Series One minis as well, although one of them didn't have the text in English so I had to figure out what the story was about from the pictures.
I actually missed out on a fair chunk of the canon from the first two series, which is why the Powersword in my Grayskull trilogy is not a blade split in two - I didn't know about that story!
I also didn't know about Trap Jaw and Tri-Klops' background either, although now I do, I'm disappointed that the MOTUC bios for them don't match those original ideas.
It's probably a bit strange for someone like me to say the Powersword legend is not part of my interpretation, but even though it is a big part of MOTU canon, it's not a big thing for me.
I prefer to think of the Powersword as a unique weapon that only has power in the hand of the one destined to wield it. So sorry, Skeletor, yours isn't half of the key to Grayskull in my book!