"Yuuki o Uketsugu Kodomotachi e" is another memorial theme, this time for the end of Frontier, and featuring darn near everyone who ever recorded a theme song or battle theme for Digimon up to that point.
Then there's "The Last Element", which played in Frontier as the Hyper / Unified Spirit Evolution theme (in other words, for Emperorgreymon y Magnagarurumon), and during the birth of Susanoomon. The track which replaced it in the dub is just as good.
On a more solemn note, Theme of Darkness ~Kouichi's Theme - while the other "Theme of ____" BGM tracks are instrumental arranges of their respective characters' image songs, Kouichi's is a slow arrange of Kouji's "Theme of Light".
The most hilarious song ever in Digimon, Get the Biggest Fire, where the two main characters of Digimon Frontier try to sing a new theme, starting with the earlier themes. They sing very badly.
Frontier's opening sequence "Fire" is so fun and uplifting. The English opening is really catchy and powerful too, to say the least.