Norse myths are kinda weird about this, 'cause the Ragnarok stuff isn't like a clear-cut team roster. You get the major Aesir like Odin and Thor front and center, obviously. Odin gets swallowed by Fenrir, which is a huge deal. Thor fights Jormungandr and they kill each other, which is maybe the single most iconic duel. But then you've got Loki, who's technically a Jotun but kinda adopted, leading the bad guys against the gods he used to hang with. His kids are the ones causing most of the havoc. Freyr dies fighting Surtr because he gave away his magic sword, which always felt like a tragic footnote. Honestly, a lot of the other Aesir and Vanir just get named as casualties; the Prose Edla is like 'and then this one died, and that one died.' It's less about a pantheon-wide battle and more about a few key, world-ending matchups between specific entities.
The biggest roles, to me, are the destructive forces themselves: the wolf Fenrir, the serpent Jormungandr, the fire giant Surtr, and Loki as the catalyst. The gods are reacting to these primal threats. Heimdall and Loki kill each other too, which is a massive moment, but it's often overshadowed by the Odin and Thor stuff. Vidar avenging Odin by tearing Fenrir's jaw apart is a huge symbolic act, but he's not a major player until that exact second. So yeah, biggest roles: Odin, Thor, Loki, Fenrir, Jormar. The rest are part of the tapestry, but the story really hinges on those five.