After so many Simoun related videos, what more can I say about this series?! It's a modern day masterpiece, with the best, most perfect soundtrack I've heard since Chance Triangle Session. It's a bit biased of me to say a soundtrack's perfect, but when it comes my persona favorites like Sailor Moon, Revolutionary Girl Utena, Fushigi Yuugi, etc. all anime with flawless music that you never get tired of.
The title of this piece is Youen Naru Kizuna no Hibiki. It plays often throughout the series, mostly during prelude esque scenes, leading up to a key moment or event. It reminds me of tango music, except more subtle. The melody is precious, just close your eyes and you'll drift away. Imagine piloting the Simoun with one of the girls you adore. ^_^
Remember, Simoun are Divine aircraft! :P