My favorite theme is Lost Emotion.
As far as arranges go, I don't really listen to many of them but Jynx's The Inevitably Forbidden Game is one of my favorites.
For original fan themes it's really tough, but I think I have to say Beneath a Deep Vanishing Cloud ~ Mow Down from Len'En 2 is my favorite, with Life Guiding Song ~ Prospect Mirai from MPP being a cloase second.