Here's a breakdown of different ways to look at it:
1. Most Disney *movies*:
* Alan Menken has composed music for many Disney films, including *The Little Mermaid*, *Beauty and the Beast*, *Aladdin*, *Pocahontas*, and *The Hunchback of Notre Dame*.
* Hans Zimmer also has a large catalogue of Disney scores, most notably *The Lion King* (1994), *Gladiator*, *The Thin Red Line*, and *Pearl Harbor*.
2. Most *songs* on Disney soundtracks:
* It's a close race between Alan Menken (again) and Richard M. Sherman and Robert B. Sherman (the brothers who wrote songs for classics like *Mary Poppins*, *The Jungle Book*, and *The Aristocats*).
3. Most *performed* Disney songs:
* This is trickier to quantify. Popular artists like Whitney Houston (who sang the iconic "I Will Always Love You" for *The Bodyguard*), Barbra Streisand (*Beauty and the Beast*) and Elton John (*The Lion King*) have performed iconic Disney songs, but many other artists have covered Disney tunes over the years.
4. Most *original* Disney songs:
* Susan Egan voices the character of Megara in *Hercules* and sang many of the film's songs.
* Lea Salonga is another strong contender, having voiced Jasmine in *Aladdin* and Mulan in *Mulan*.
Ultimately, there's no single answer to your question. Depending on how you define "sings" and "Disney song," the answer could be a composer, a singer, or a collection of artists.