Le' Miserables' by Victor Hugo has always been a literature-play classic. However, for me, the musical-play that surpasses the timeliness of Le' Miserables' is Cats.
Cats has been one of the most successful plays which was able to beautifully merge music, acting, and acrobatics in one. I remember my Grade 6 days wherein we presented Cats back in our school. It was a lot of fun projecting like stupid cats (I'm not good in acting so why did they have to choose me?)
The best songs in Cats are the opening song Jellicle Cats and the Mr. Mephistopheles' character song. Of course, the song Memory (Grizelda's character song) brings out the tear in anyone who will listen.
I haven't watched many musicals, so saying this is my "favorite" is, I suppose, a bit of a stretch. But, I just watched WICKED and boy was it awesome! Didn't watch the "original" at Broadway or anything, so it was a different cast, but I thought they were great! If Broadway is even better than this, I am so watching Wicked at Broadway the first chance I get. I loved the acting, the singing, the costumes, the choreography, the set... Just everything! Some critics say it didn't link up well with The Wizard of Oz, but well, I can't really remember the details from The Wizard of Oz anyway, so that didn't affect me at all It was great as a standalone story, though of course you'll recognize characters from The Wizard of Oz and it's nice to see how they came about (eg the Tin Man and the lion).
My favorite songs were definitely the finale of Act I - Defying Gravity, and For Good from the end of Act II