I'm a bit late to this game but my money is on Barbra and Hello Dolly!

There are several places throughtout the season where Emmett refers to Barbra films

"Money is like manure..." Hello Dolly

"Put on your Sunday Clothes..." Hello Dolly

"Like Barbra in the Mirror has Two Faces..." The Mirror has Two Faces

Plus you have the following to choose from:
Funny Girl
Funny Lady
Hello Dolly
On a Clear Day
Yentl
A Star is Born (remake)
The Way We Were (not really a musical, but referred to in the West Side Story episode)

OTOH, we have seemed to forgotten the obvious - Judy Garland movies, starting with:
The Wizard of Oz - Somewhere Over the Rainbow (need I say more?)

That is so funny you said Wizard of Oz! I was thinking that because of Judy Garland (the more I get comments on this post, the more I realize I do know more musicals than I thought) but then once again, I had to think in terms of Emmett's most favoritist-favoritist (is that a word?) musical because it's something he has to be fanatical about.

Also, it can't be too dark and let's face it, Wizard of Oz was actually a dark movie. The fic itself is already dark so you need something that's in-your-face fluffy and bright. I think I might go with Funny Girl or Hello Dolly because of all the Barbra references in the show and it just fits.

Thanks!