1) Les Miserables - I saw Les Mis for the first time in high school and was totally hooked. Even my husband, who in general doesn't like musicals, likes this tale of Jean Val Jean, convict 24601. My favorite song has to be On my Own. I love the original London cast, but unfortunately I can't seem to find a link to that version and nonw of the rest sound nearly as good. Another good one is Do You Hear the People Sing. You can see a good clip of that by clicking here.
2) Sweeney Todd - I saw the Angela Lansbury version of this great musical for the first time with my friend(s) in a viewing room at the library in college. Ever since then the musical has intrigued me. Johnny Depp didn't hurt my liking of the musical either :-) Favorite songs = Green Finch and Linnet Bird (one that a friend was very good at singing in high school) and The Worst Pies in London (both Angela Lansbury and Helena Bonham Carter did great versions).
3) Phantom of the Opera - The first big musical theatre production that I recall seeing. I know I saw many on the small stage, but I saw this at the Wang and it was a big deal. I spent much of my free time in high school striving to sound like Christine along with my other friends. As I recall the cable access hallway had some damn good acoustics! Favorite songs include Think of Me and Phantom of the Opera. You can see a good version of the former here.
4) Wicked - So, I've never actually seen this musical. I read the book and listened to a few clips on the music online and then just downloaded the soundtrack, knowing that I would love it. And love it I do. I hope some day I can actually see a performance. My favorite song has to be Popular which you can see here.
5) Little Shop of Horrors - This is such an awesome musical! The movie starring Rick Moranis is hilarious. Steve Martin plays the dentist and her is absolutely perfect. Favorite songs are Skid Row, Suddenly Seymore and Dentist.
6) Dr. Horrible's Sing Along Blog - Not a standard of musical theatre, but a musical nonetheless. Neil Patrick Harris, Nathan Fillian and Felicia Day are wonderful in this web musical. Favorite songs include Bad Horse Chorus, A Man's Gotta Do and Brand New Day. Just download the whole thing now and get it out of the way.
7) Once More with Feeling - Joss Whedon's musical episode of Buffy the Vampire Slayer sung by the actual cast. Some are better singers than others with Anthony Stewart Head and Amber Benson showing their vocal chops off the best. Favorite songs include I've got a Theory / Bunnies / If We're Together which can be heard here, Walk Through the Fire and Under Your Spell.
8) Nightmare Before Christmas - Tim Burton combines Christmas, Halloween, claymation and singing with great results. Favorite songs include This is Halloween and Sally's Song.
9) Bye Bye Birdie - The version I'm most familiar with is the 2005 tv miniseries starring Jason Alexander and Vanessa Williams. The story is of the rock legend Conrad Birdie being drafted into the army and how his adoring (and squealing) fans say farwell before he heads off to war. There are quite a few excellent songs but my favorites are Bye Bye Birdie (you can see a clip here) and Spanish Rose.
10) Newsies - Ah.... gotta love those Disney musicals! This was another high school obsession for me. I adored Bill Pullman and musicals, so when I heard he sang a few lines in this I most definitely had a squee moment. The movie is the story of Pulitzer and Hearst raising the cost of their papers by a penny and how it affects the newspaper boy that were trying to scrape by a living. It is wonderful and stars a very young Christian Bale. My favorites from the movie are King of New York and The World Will Know. Below you can see the latter: