Hibari's Theme

Not exclusively used by Hibari later on, but it always serves as a reminder of how dangerous he is.

My favorite instance of it is when he's about to beat the shit out of Yamamoto (I think) for doing something suspicious, but is calmed down before he can let loose. So the piano part of his theme plays, but cuts off right before the rest of the orchestra can enter.

Nice story/music integration there.

Holy War

A more intense version of Hibari's Theme that's used a lot in and around the Varia arc. The band nerd in me loves the strong low brass here that adds a sense of urgency and seriousness that wasn't as strong before this arc.

Tsuna Awakens

It's very "Rocky"-esque, but with the orchestral flair that the KHR soundtrack does so well instead of 70s funk.

And the key modulation in the middle really adds to the energy. I just wish the whole thing was longer than like a minute and a half! The far and away best usage of this song was during the Tsuna vs Mukuro battle in Kokuyo, at the first appearance of Hyper Dying Will.

You can tell how pumped I was when I saw that, and the music did a lot of the work.

Smile For You

Okay, this is just a pet one that I like. It's cute, what can I say? Boo on me for not liking most of the OPs and EDs, but anything that captures the essence of Pretty Cure/Hamtaro/similar Japanese cuteness gets an A in my book.