Though it may not be the most difficult, the piano part to By Threes from I Love My Wife by Coleman is a glorious spotlight for the pianist in the show. The piano break is reminiscent of Gershwin. I marvel every time I listen to the recording.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian
Man, this thread makes me miss piano playing. I never got through all of the songs in my copy of Wicked; "Popular" and "I Couldn't Be Happier" were the easiest for me. Les Miz was pretty fun, too.
Though it isn't difficult, "Wick" in the Secret Garden vocal selections gives the pianist a great workout. The accompaniment is in the form of a toccata, so you are tearing away non-stop from first measure to last. Very satisfying.
I once performed "I've Grown Accustomed to Her Face" (the full five minutes-- not just the song), and it gave my pianist some difficulty and that's 1956. We also had a over-zealous Nazi director for the cabaret we were doing who liked to pick tempos for me/her, until I told him I'm doing it my way (and the way Rex Harrison does it on the album) and he can go shove it. (I didn't say "shove it", but I wasn't sweet and pleasant either.)
The published vocal score of GUYS AND DOLLS has a lot of accidentals and weird keys in it, and I've heard a few pianists complain about it. Perhaps a the Vocal Selections folio is less complicated.