Senna has been used countless times to add huge dramatic effect, usually for a sudden strike or the arrival of a powerful figure. It works. Every time.
Senna's Theme, by the way, is derived from a violin adaptation of a portion of Bach's Toccata and Fugue in D Minor (1:12 to 1:24).
Speaking of theme songs, "Ichirin no Hana", debuting with a broadcast special consisting of 2 awesome episodes, also falls squarely here.
"D-tecnoLife" was an excellent introduction to the second season.
"* ~Asterisk~" by ORANGE RANGE. The OP for the first season of the show. Also, "Houkiboshi" by Younha, the third ED.
"* ~Asterisk~" went to top of the charts the second it was released in 2005, and stayed there for 22 weeks. Now if that doesn't say something about how awesome that song is, nothing will.
Kubo chooses theme songs for most of the important characters.
Ichigo - News From the Front by Bad Religion, Chad - No Song Unheard by The Hellacopters, Isshin - Don't Drag Me Down by Social Distortion, Ulquiorra - Moonshield by In Flames, Grimmjow - Step Inside by A Bombs, and Kyoraku - Por Una Cabeza by Carlos Gardel to name a few.
The first ending theme, "Life is Like a Boat". It's just beautifully sad-sounding, but ultimately still hopeful.
Never Meant To Belong is extremely heartwrenching in a beautiful way. It has just enough of a hopeful feel to not feel cheesy, while the well-orchestrated instruments make this one a winner.