I will definitely agree with Wicked (original cast preferred, although I wouldn't mind Hilty as Glinda) Les Mis and Rent and of course the Lion King.
The reason why the Lion King is so awesome is a combination of just about everything. Music, costumes, everything just works so well and is so comprehensive in detail it all just blows you away.
West Side Story is another one of my favorite ones when done correctly. Too often I've heard a very lackluster "America" where it sounds like the cast is being forced at gunpoint to sing it instead of the lively spirited dancing number it is supposed to be. Some great vocal numbers by Bernstein however, with "Maria" and "Something's Coming" as well as "One Heart" (or at least I think that's the name of it).