Mother. Many people say it has a legendary, unforgettable sound track. However about 30% of the music are real life songs that are good. The other 70% is original and amazing.
Like many other people, I like the DKC soundtracks. I also like the Goldeneye (64) soundtrack, particularly Surface 1, Surface 2 and Caverns. I also quite like the Turok 2: Seeds of Evil soundtrack, simply because almost all the music, barring menus, boss battles, etc, are extremely long (ranging from just over 5 minutes, to er.... well, I gave up after 5 hours, trying to work out when Oblivion loops, but it's ... well over 5 hours)