At least in my experience, Judy sings by far the most frequently. I've had her a few times during my play through, and she's constantly out somewhere busting out a tune.
On my island, Cherry, Roscoe and Kabuki sing the most often in the most random places. Marshal and Judy also sing quite often and Judy even held a concert in the plaza once ☺
I personally love it when I catch a villager with a different hobby starts singing randomly. One time, Poppy was singing for hours in the plaza and I just put it as my background music the entire time because it's too precious
marshal is my island's pop star even though ruby claims to be a pop star herself.
i swear the marshmallow boy sings almost every time i see him and there's honestly no complaints from me.
the Skye I have is the one who sings the most on Seastar as well! She's almost always singing. For a while Kabuki was singing most but then Skye moved in and took over singing I guess? It's adorable!
Avid singers on my island are Pietro (obviously ) next to that I'd actually say Kabuki which still surprises me to this day. I managed eventually to get all of mine singing but most are lesser spotted singers).
Rosie and Pancetti are my little vocalists. I have no idea if either of them have the music hobby, but Rosie will usually sing along with whatever song is playing on the radio outside while Pancetti will just freestyle.
A lot of people are mentioning Marshal as well, but I haven't really seen him sing on my island yet. Maybe he's shy?
Most of my villagers have music for their hobbies. I didn't plan it that way, just worked out with who I wanted on my island.
Out of all of them, I'd say that Cherry, Gruff, Lolly, and Sally sing most often, with Cherry and Gruff being the most vocal of the four. The other villagers will join in, but those are my most musical.