Another vote for Spelling Bee, here! Simply genius. I'd also like to throw out:
--Avenue Q (also genius)
--The Book of Mormon (I'm pretty tired of it now, but when the jokes were fresh, they were really excellent!)
--Gentleman's Guide (funny in a different way - more clever and witty than outright hilarious)
--Spamalot (gotta love that Jew song)
--Xanadu (laughed myself silly!)
--[title of show] (not as funny as the others on the list, but still very funny)
I'd also like give a shoutout to One Man, Two Guvnors - a play with music. I've literally never laughed harder in the theatre in my entire life. So funny!