Summary
Possibly due to its poor reception, the music of Sonic and the Black Knight sadly hasn't gotten as much attention despite its mix of "veteran" Sonic composers (including--among others--Jun Senoue, Richard Jacques, Fumie Kumatani, and Howard Drossin).
The game's "otherwordly" setting proved an able excuse for themes of the past such as "Throw it All Away" and even "My Sweet Passion" to return. Unlike games such as Sonic Generations, however, this fan service isn't the entire OST. Actually, most of the music is new and takes on theme of its own.
This game would be the last to feature "the Sonic band" Crush 40 for a long time, but they did make quite a few themes this time around, including both the opening and closing theme, as well as a cover of "Seven Rings in Hand" (the main theme of Sonic and the Secret Rings).
I would also like to mention that the cutscene music is pretty epic and dramatic rather than filler like in some of the games. "Merlina: Queen of the Underworld" is one of my favorites. Interestingly a few scenes seem to be inspired by the "World Adventure" theme of Sonic Unleashed.
There are also some very addictive alternatives, like the acoustic version of "Knight of the Wind" and the "Fan Art" version of "It Doesn't Matter."
Best Song: Dragon's Lair
Runner-Up (s): Misty Lake, Deep Woods, Molten Mine, Crystal Cave, Titanic Plain, Knight's Passage, Shrouded Forest, The Caudron, Dragon Slayer, Far Away Avalon, Battle Menu, 5 Stars, Name Entry, Gallery, Blacksmith, Fan Art, Theme of Merlina, Calm After the Battle: Arondight, Showdown with King Arthur, Merlina: Her Wicked Smile, End of the World, Escape from Destruction, Into the Dark Hallow, Excalibur Sonic, Live Life Merlina and Sonic, all rival characters appearance music
There were plenty of good options, and Molten Mine and Shrouded Forest were certainly fair possibilities. Even though it's cutscene music, I was even considering Merlina's theme, because I love the piano. Sometimes, though, epic guitar and drums just win out.
Dragon's Lair is similar to its nearby stage, Crystal Cave, but more intense. The song starts edgy, a classic Sonic song. Then heightened up with drama, it becomes the mix of speed, tension, and heroism that matches the game so perfectly.
Worst Song: Great Megalith
Runner Up (s):
It's not that bad, and it gets better as it goes on. Still strong but repetitive and gets old fast.
Best Vocal Song: Live Life
Runner-Up (s): With Me (Both Versions), Knight of the Wind, 7 Rings in Hand (Crush 40), Through the Fire, Fight the Night
100% convinced this is the best closing theme in any game. Probably the best Crush 40 song, too. As its title suggest, it's a reflective song about having potential and living life despite its inevitable end. While done in a happy tone, it can be quite sad. The other options were great, too, but "Live Life" remains my favorite.
Worst Vocal Song: 7 Rings in Hand (Fairytales in Trance)
Runner Up (s):N/a
I always forget about this one. It's a more relaxed, chill version of the song that really isn't bad, but not as good. For whatever reason, it strikes me more as "Silver and the Secret Rings."
Final Ranking: S
Like with Sonic and the Secret Rings, this music is stunning. This title, however, is more serious and generally a step above. Like with Sonic Unleashed, it goes beyond even Sonic standards.
Its game exclusive Celtic influences make it unique and memorable, but at the same time I think that being so tied to "the Black Knight" type of sound rather than just the "Sonic" type of sound keeps it from being the best OST (yet between the BMG and various vocal themes, it seems pretty close).