I always go for the 'marrying the love of my life, hiccup' moment from the finale. It's not the most action-packed, but the emotion is so huge and it's the perfect payoff for the whole trilogy. The outfit is basically Hiccup's chief gear, which is a great, recognizable look but also pretty manageable to put together compared to some full dragon-scale suits. You get to carry a little wooden Toothless too, which is an adorable prop.
What really makes it special for cosplay, though, is the interaction. You stand there with your partner (or a friend as Astrid) and that line just lands. It's a moment of pure joy, and at cons, people who get it will absolutely light up. It feels less like just wearing a costume and more like sharing a specific, cherished feeling with other fans.