I was one my phone when I saw this, and I just cracked my knuckles and went "Right. We're gonna need a PC for this one!" 😅
DISCLAIMER: Other people have said a lot of this stuff before me I expect, but I'm just not going to read their comments until I've finished my own so I can approach this task with fresh eyes. This is literally the kinds of things I write about in my free time (I know, my life's so exciting)!
Biggest Act of Betrayal: Of course I'm going to have to say his lies to Thor about Odin's death. It depends, of course, what you mean by biggest though. This definitely wreaked the largest psychological and emotional impact of any betrayal he orchestrates, hurting both himself and those around him. Loki was feeling so emotionally distressed himself, that he let pure selfishness guide him. A particularly significant moment happens when Loki turns away from Thor and leaves: his face is pained, and no matter how much he wants what he's getting, it clearly doesn't spare him any guilt in the moment, to see his brother in such a state. Perhaps he thought that it would be a more triumphant moment than it was. (Confession, the whole scene makes me cry almost every time.)
However, if you mean his "biggest" betrayal with the biggest scheme and longest played game behind it, it was obviously his faked death and usurping of the throne- it was such a long-played betrayal, that Thor didn't even find out until years later.
Most heroic moment: There are two that I really struggled to decide between with this one: namely, Loki's protection of Jane in Thor: Dark World, and of course his stabbing of Thanos. I have a winner now, but I'll go over my thought processes for them both (I know, I know I'm annoying).]
When Loki saves Jane, it's completely uncalled for, and that's what makes it so surprising. She's a random Midgardian girl who he met less than a day ago. (And who gave him quite an impressive punch in the face, I might add.) And this, the fact that he saves her twice despite previously mocking Thor that her lifespan's short anyway- it's a fact that's truly significant to Loki's character. He's not the cold-blooded killer everyone thinks he is- or that he likes to think he is. Loki builds up this image for himself, of a villain who has no feelings and prizes his own life above everyone else's- and a significant proportion of the time, he manages to achieve this. But on this occasion, he does something truly unselfish- and part of me wonders if this is what inspires him to go onto another betrayal- although maybe I'm going too deep. Again.
Honestly, I would love to see Loki interact with Darcy. I can already imagine the comments Darcy's going to make on him, and equally the ones he'll make on her. Not for a very big scene or anything, but I just always though it would be pretty hilarious.
On a side note, I really would rather not see Loki get a love interest. I've written articles on this, and his character just doesn't need it to grow. It would be both superfluous and detract from the love interest's overall character, as a character who's introduced purely as a love interest from the start takes a lot of redeeming, in my opinion, for them to actually have a personality that's not just "pretty".