I don't agree that Raoul is a thankless role. He gets to sing a well-known number ("All I Ask of You"
and many Raouls have gone on to play the Phantom. To me, the thankless role in POTO is Ubaldo Piangi. If he's played solely for laughs (as in the touring version), he becomes obnoxious. If he's played straight, the story still implies he's incompetent as a singer. Also, his Hannibal costume weighs something like 70 pounds.
I don't think Cosette is thankless, either, because she's the only female in the story who isn't expected to belt. As a result, her operatic singing stands out. Believe it or not, in some ways, I think Javert is thankless. No matter how fabulous is the performance of the baritone playing him, the role is overshadowed by Jean Valjean.
I think Colonel Pickering is thankless in "My Fair Lady." It's just not a meaty role and everyone else overshadows him.
Audrey
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.