I like Genie's list.

Here's some more. "All The Things You Are" by Jerome Kern.
"Stella by Starlight" by Johnny Mandel
"Over the Rainbow" Yip Harburg and Harold Arlen
I like "Lagan Love" and "Lark in the Clear Air"

Almost any tune by George Gershwin and Cole Porter. Hard to say most beautiful here.

"There's a Place for Us", "Tonight" and "Maria" from Westside Story are right up there.

I'm biased but i wrote a beautiful tune set to Yip Harburg's poem which I hope that someone, some day will hear.