Openings and Endings can really define an anime, but the background or insert songs during the anime can really shape it. List some of your favorite background ost or anime series that have good background music.
Oh man, this post was just made for me to comment on.
I've been a huge fan of the Dragon Ball Z anime so I'll list some of my favourite background themes.
1) Gohan goes SSJ2 (Japanese version)- This song brings out the chills.
2) The Dragon theme (Funi Dub)- One of the greatest anime themes ever made in my honest opinion. This one played during the Kamehameha battle, during Vegeta's Final Flash on perfect Cell and of course, everytime someone summones the Dragon.
3) Psycho-Pass theme- I don't know what this theme is called but I'll link it (https://www.youtube.com/watch?v=nArpKw8ObM0). This theme was played at the PERFECT moments during the anime and that's what makes it so special for me.
I really don't know why but the theme songs from Macross: Frontier always makes me happy,
The song actually starts at the middle titled "How 'bout my star", it has a dozen or so songs which are really uplifting.