I haven't played the Pokemon Mystery Dungeon games in a long time, but I do recall liking the Steam Cave BGM and the Craggy Coast BGM from Explorers of Time & Darkness.
"Steam Cave", "Upper Steam Cave", "Northern Desert", and "Primal Dialga" are my preferred BGMs in the PMD series. Though "Mystifying Forest" is the most memorable out of all of them.
I always mix up Mystifying Forest with Miracle Sea's Track, because the two are essentially the same with some tone and speed differences.