As you may know, the "piano keyboard" design of the melodica nowadays is actually from Japan. Hence quite a lot of the very best melodica players are from Japan.
Tokyo Melodica Orchestra is definitely one of the very best melodica group I have ever heard.
I would actually call this group as "Melodica Barbershop" rather than "Melodica Orchestra" though, as they are not a group that has like 30 to 50 members, they are actually 4 players, two playing the alto, one plays the bass and one plays the lead. So it is basically the same as "barbershop music".
Anyway here is a video of them playing Gershwin's "Rhapsody in Blue", enjoy!!