I think 'You know what they call a Quarter Pounder with Cheese in Paris?' could also be considered very iconic. It's such a simple yet effective line that shows the cultural differences and the characters' banter in a really cool way. It's a line that many people remember when they think of Pulp Fiction and Samuel L. Jackson's character in it.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Yes, the song was Weaken Me Up, sung by a mixed band. The song was released in 2015. It was a fast-paced, energetic song with lyrics that touched on the theme of love, strength and courage. The song was very popular on social media and was seen by many as a very inspirational song.