As someone who has enjoyed Spamalot hugely in the past year, I have to say I'm surprised not many people have mentioned 'Not Yet Dead' as their funniest song. Esp Christian Borle! Maybe it's funnier on stage than on the soundtrack (and it's pretty early on in the show)? I also love 'The Song That Goes Like This', 'Find Your Grail', 'All For One' and 'You Won't Succeed on Broadway'.
Of the other Broadway musicals I've seen (admittedly a limited number compared to the West End), my faves have been 'Great Big Stuff', 'Springtime for Hitler' 'All About Ruprecht' and 'Dance 10, Looks 3'.
After reading this thread I'm looking forward to seeing Avenue Q and Spelling Bee when I return to NY. (Hopefully! As well as Curtains and The Drowsy Chaperone).
Seen some shows in my time....