The casting of Idina Menzel as Lea Michelle's onscreen biological mother is brilliant. There is little that could make the episode where they sang I Dreamed a Dream from Les Mis better than the fact that Doogie sang Aerosmith's Dream On with Will Schuester.
As an aside.. the show lost me about halfway through the first season. I just couldn't get hooked. They reeled me back in with the Madonna episode and now I'm hooked again. I may even buy the CDs.