As a music major, this clearly is the most important category for me (at the Tony's at least). The score can make or break a show.
1. Light in the Piazza most certainly deserves the top spot. Stephen Holden got it right when he said in the New York Times that it has the most romantic score since West Side Story. The music is just beautiful, and composed so well down to every last note by each singer and instrument. Even a song like "The Joy You Feel" is compositionally brilliant. The music serves to capture the feeling and thoughts of the actors when they can no longer be expressed in words. One needs only to listen to the full overture on the CD to understand how beautiful this music is.
2. Urinetown comes in a very close second. On first listen, most might think the score to be a bit trite, silly, or just fun, but nothing substantial. What is so great about this music is just how much Mr. Hollman was able to channel and almost sound exactly like the Brecht/Weill style from the Threepenny Opera, yet sound so different at the same time. Just listen to the overtures of both shows and this immediately becomes clear. The lyrics are extremely clever, and the music is relentlessly inventive. The Act I finale is so complex and reaches an almost operatic height at some points.
3. If we are basing this solely on music, the next would probably be Spring Awakening. As a rock score, it's is extremely successful combining different styles of rock that somehow manage to sound coherent. Mr. Sheik created a score that was tap-your-toe good (that's a bad adjective) without pandering to the audience nor compromising his own skills as a musician. He also managed to make a rock score have a Broadway sound, which makes the score even more impressive. I'm not in the camp of those who think the lyrics are awful....but they sure ain't Sondheim.
4. Hairspray is in the same mold as Spring Awakening where a rock n roll type score manages to sound at home on the Broadway stage. The score is fun, and yes at times fluff, but it is extremely smart music. The scpre ios also extremely diverse..from extremely inspirational (I Know where I've been), to pastiche (You're timeless to me), to thrilling (You can't stop the beat). Mr. Shaiman's scores have never failed, and prove to be smart but at the same time fun and extremely accessible (on a side note, the songs from the South Park movie are amazing!)
5./6. It's hard to choose between Avenue Q and the Producers, but I think that Q has the slight edge. Both are extremely funny, clever musical comedies, but musical commedies at the exact opposite end of the spectrum. Clearly, The Producers was a musical comedy throw-back of the days of yore, while Avenue Q is just another exmaple of an evolving art form. The Producers has an extremely clever and well-thought out score that harkens back to the musicals of the 60s. The music is not neccessarily beautiful or exiting, but it serves the show and the actors extremely well. I think the orchestrator deserves a lot of credit on this one as Mel Brooks himself has stated that he pretty much justs hums a tune down, maybe plunks it out on the piano, and has someone with compositional or orchestrational experience expand and flesh out the melodies and songs he writes.
Avenue Q has an extremely clever, hip, relevant score that is laugh out loud funny. It has a modern sound to it, but it's most certainly a score that works for the stage. It is not a substantial work, nor was it meant to be. The CD is a blast to listen to, and whether or not it deserved to win Best Score is an issue in itself.
7. The Drowsy Chaperone. The question here is how do you make a good score out of something thats suppossed to be bad and cheesy at the start? This becomes a problem at the start. That being said, the score is still fun, big, and brassy. There are some great numbers in the show, and talk about great tap-your-toe fun music. It most certainly will not go down as one of the better scores in history, but in itself, it's a great piece of fun fluff.
8. Aida is the only one of the 8 shows that I haven't seen, therefore, I can't quite judge how well the music served the show...or the advacement of the plot. (The opera Aida, however, is a completely different story). That being said, being a huge fan of Elton John's music, I don't think he was very successful with this show. There are some songs that are fun, and well written, but most seemed to be overblown and over-done. A rock score that didn't quite seem or know whether it wanted to be rock or Broadway.
Allllll that being said, I think that Caroline, or Change was simply ahead of it's time. The score encompasses so many different styles, and utilizes the the operatic/Wagnerian techniques of themes. Each character has their own theme/entrance music, and melodic style when they sing. The different styles of each character, human or not, fit perfectly. I think many were turned off by the "sophistication" of the piece. I do believe in time, most will view this score with completely and total admiration and respect. Ms. tesori also showed that after a standard score such as Millie, she was able to write a score with such amazing depth, heart, complexity, and variety.
As for Grey Gardens, I will say that they lyrics are better than those in Spring Awakening. However, I don't understand those who think it's score is leaps and bounds over Spring Awakening. The score is too inconsistent for it to be considered a "masterpiece." For every great song that transported you (Will you?, Around the World, Another Winter), there were songs that we simply subpar (forgive me, but i cannot remember the titles of the songs, but most were in the first act of the show). Spring Awakening's score is at a high level of composition consistently throughout the piece, and while a song like "Will You" may be just as good as or even better than any song in Spring Awakening, a few songs does not a Best Score make.
Phewww...that is all!
-There's the muddle in the middle. There's the puddle where the poodle did the piddle."
The Light In The Piazza (Adam Guettel) 
