Okay, hello. This is a huge question. I am going to have a panic attack if I keep thinking about it being definitive, so I will just flow with it like jazz.
First of all, The Music Man is horrible. I'M NOT SAYING I DON'T ENJOY IT. But it's not good. There's a song and accompanying garish dance number in the second act called "Shipoopi." Again: just saying. Do I thrill when that gayest of all gay men that ever darkened a doorway (not a euphemism), Robert Preston, admonishes River City about the trouble it's got? Of course I do. But is it a good show? Define "good."
Here are ten fantastic shows, four of which I'll allot to Stephen Sondheim, the Picasso of Musical Theater in that the lay-listener not think his melodies are pretty, but all who are even semi-learned in the matter must concede that he's responsible for singlehandedly modernizing the artform.
Essentials of the Sondheim Canon (my favorites)
Assassins
Sunday In The Park with George
Sweeney Todd
Company
I am not the world's biggest R&H fan, but I appreciate them. I'll cast my vote with Oklahoma! as my fave of their oeuvre, but I'm not putting it in the top ten. MOVING ON!
I will add
Guys & Dolls
as well as
Gypsy
here, because they are timeless Americana, and Gypsy is arguably the best musical of all time; also a Sondheim effort, but collaborative.
What do I have, four left? Hedwig is great. If you like Hedwig, and you're in NYC, get yourself tickets to see Passing Strange. The book packs a wallop, and I swear you will love Stew's songs. I will, however, hedge my bets on Jesus Christ Superstar as my pick for rep rock opera on the top ten.
Final three, huh. Well, assuming these are shows and not films (that's a whole other list), I have to include A Chorus Line, my only dance-heavy pick. And I'll do two modern shows just to show that theater's not dead, despite Duncan Sheik's Yerba Mate-fueled efforts. I'll include The Drowsy Chaperone, which is crisp, smart and actually funny, not just "theater-funny," and David Yazbek's adaptation of The Full Monty, which imported the heart of that film from working-class Britain to working-class Buffalo, lyrically articulated the confusion between Carly Simon & Carole King, and had full frontal male nudity at the end of the show, which is more than you can say for Jersey Boys.
THANK YOU FOR ASKING ME THIS QUESTION!