Game: Touhou 7: Perfect Cherry Blossom
Platform: PC (plus a DS port if you run homebrew :3)
Song: Bloom Nobly, Cherry Blossoms of Suziyome ~ Border of Life
Reason: It is a truly beautiful song, and plays perfectly with the timing of the boss battle against Yuyuko. I feel that it really sums up the series... playful, yet elegant.
Game: Touhou 5: Mystic Square
Platform: NEC-98 (PC if you can find an emulator)
Song: The Grimoire of Alice
Reason: This song is also elegant, but it feels more climactic. It gives a nice sense of danger as well, as Alice under the control of the unsealed Grimoire is one of the few people that is able to destroy Gensokyo (the series' world).
Actually, let me post a lot of the good songs from Touhou Project that I know the names of...
Please forgive any misspellings
Love-Colored Magic
Bad Apple
The Grimoire of Alice
Beloved Tomboyish Daughter
Sepptite for the Dead Princess
U. N. Owen was Her?
Crystallized Silver
The Doll Maker of Buscechi
Doll Judgement
Phantom Ensemble
Hiaorai Shoots a Strange Bird ~ Till When?
Ultimate Truth
Bloom Nobly, Cherry Blossoms of Suziyome ~ Border of Life
Necrofantasia
Love-Colored Master Spark
Lunatic Red Eyes
Voyager 1969
Gensokyo Millennium ~ History of the Moon
Voyager 1970
Reach for the Moon ~ Immortal Smoke
Eastern Youkai Beauty
The Adventures of the Love-Struck Tomboy
Wind God Girl
The Gensokyo the Gods Loved
Faith is for the Transient People
Native Faith
Green-Eyed Jealousy
Solar Sect of Mystic Wisdom ~ Nuclear Fusion
And that's all I can remember off the top of my head... Go search any of these and listen to it. You will be amazed.