2. Idina Menzel as "Shelby Corcoran"
Idina Menzel began as a celebrity guest star in the first season, was absent throughout the entire second season then had a huge role to play in the third- before disappearing without explanation never to be seen again. Out of all the celebrity guest stars, Idina Menzel certainly had the most important role to play as Rachel's biological mother and the scenes in which the mother and daughter share together are certainly some of the most touching and well constructed scenes on the entire show. It baffles me why in Season 3 they decided to have Shelby spend all of her time with Puck as apposed to her daughter Rachel who she only shares one duet with, and not that many more scenes with after this. Regardless, as a Broadway star famous for her role in the original production of Wicked, Idina Menzel certainly has the most impressive voice out of all it's celebrity stunt casting (save for the actress who features at number 1) and a voice as big as hers was needed to not only give some explanation to Rachel's exceptional talent, but also so she could hold her own in the three duets she shares with the character "I Dreamed A Dream", "Poker Face" and "Somewhere". This is a character I would love to see back sometime in season 4, but at this point that seems somewhat unlikely.