The
Sonic the Hedgehog series wins this category for myself, hands down, no contest. Ever since the games began to decline in quality, one thing has always remained divine, and that's the soundtrack.
Heck even Sonic the Hedgehog (2006), which is considered to be the biggest disaster of a game in the series' lifetime, actually has an incredible and epic soundtrack.
What's also nice is the diversity. There's rock, rap, electronica, jazz and even classical implemented in the series' BGM. The variety of vocal themes is also good for those who don't like listening to just instruments all the time too.