Votum Stellarum (Anthem Trance), by iconoclasm (dj TAKA and wac) is pretty awesome itself, but the remix by dj TAKA later featured on pop'n 15 (Anthem Trance Remix), however, is simply incredible.
Space Dog (Space Journey) from pop'n music 11. Doubles as a Tear Jerker as it was inspired by Laika.
Moon (Sadness Euro) from pop'n music 13 CARNIVAL is one of the very few reasons Beat'n Groovy doesn't completely suck.
Pop'n Music Ron (Megane Rock) by Gira Gira Megane Dan from pop'n music 13 CARNIVAL. So popular it spawned a sequel song Ryouran Hit Chart (Negame Rock) on GF/DM V3 and pop'n music 16 PARTY and a remix by MAKI (Megane Kayo) on pop'n music 18 Sengoku Retsuden.
From pop'n music 14 FEVER, we have the awesome techno/flamenco hybrid hora de verdad (Cyber Flamenco).
In the Ruins (Ancient Euro) from pop'n Music 15, a Eurobeat track with an ancient flavor to it.
Tree in Lake (Contemporary Nation 4), from the game pop'n Music 17 THE MOVIE, yet another instant classic Nekomata track.