It can be a single or a soundtrack. I'd have to say my favorite single is Together 2007. I loved that opening (in fact it is my ringtone) as well as Oracion. As for soundtracks, I would have to say the Jirachi one is my favorite. I think the tracks for the short really stand out in comparison with the tracks for shorts from other movies, and I felt the music really conveyed a sense of the sadness Max faces when leaving Jirachi. One of my favorite tracks was the Butler's Theme. It was really dark-sounding but really cool at the same time. And to top it off I really enjoyed the ending theme as well.

My favorite cd is still the 8th Pocket Monsters Movie Mew and the Wave-Guiding Hero Lucario Music Collection (劇場版ポケットモンスター ミュウと波導の勇者ルカリオミュージックコレクション). It has some of the best and most unique music in the entire series and has been used quite a bit since AG in the TV series. Aaron's theme and Battle Frontier really make that CD one of the best one Shinji Miyasaki has ever done.

I'll have to toss in a vote for the recent Rica the Pokemon Song Best, which contains a lot of my favorite songs from the franchise.

As for favorite single? Chiisaki mono, hands down my favorite Pokemon song.