Guys and Dolls or Fiddler on the Roof. Been in the pit for both, and both were a blast.
Played both those shows and many others.
I've done "Fiddler on the Roof" three times and would do it again in a minute. Others like "Camelot" I would never want to play again, ever, ever, ever... Some like "Sweet Charity" are so completely unmemorable that I can hardly recall anything from them: music or book (OK, "Hey, Big Spender"). Some were enormous fun to play ("Seussical") but I couldn't whistle a tune from them afterwards.
My all time favourite? "Chess".
It came out on vinyl the same year as Les Miz and Phantom but was musically stronger than both in my books. Tim Rice with Benny and Bjorn from Abba. Huge fun to play—especially a concert version I played in. A bunch of keyboards (including synth strings), two guitars, bass, drums, real strings and brass, choir and soloists. It has recitativo, arias, duets, and even a quartet like European opera, and has feels that run from Verdi to rock & funk. I got to play both that concert version and a show run back in the early 90's before the endless revisions of the show mauled it beyond recognition: genius run to ruin. If you don't know the show, listen to the original record version with the cover below from 1984 on RCA.
**2nd prize for fun to play goes to Evita (played it twice as well).