This is the episode I've rewatched the most times, why? It's simple, it's just a fantastic episode with songs that are memorable, catchy and well written. The Ace in the hole here is Neil Patrick Harris, guest starring as the titular character. And he is excellent, the guy sure can sing, just as amazing and impressive is Grey DeLisle as Black Canary, she has a beautiful voice and really gets to shine here. My favorite song is probably "Drives Us Bats", but there isn't a weak song to be found here.
Furthermore there is a story here, tying all the madness together. Sure it's not deep or even interesting, but it's enough to set a frame around all the great musical numbers. Batman doesn't really sing, and when he does he's using autotune, and it's not Diedrich Bader's voice, it sounded more like Jeff Bennett.
Mayhem of the Music Meister is an absolutely unique and fantastic episode that is a lot of fun, with songs that are surprisingly great in quality. Neil Patrick Harris and Grey DeLisle steal the show and are truly talented singers.