Carrie Fisher was, hands down, the best guest star of the show's run.

This is also apparently two jokes in one (as 30 Rock is known for). From the IMDB trivia page for "Rosemary’s Baby":

When Liz meets Rosemary (Carrie Fisher) at her book signing, Liz exclaims, “You are great! I grew up wanting to be you!” and Rosemary responds, “I grew up wanting to be Samantha Stephens on Bewitched. But the closest I got was being married to a gay guy for two years.” This is a reference both to the behind-the-scenes private life of Dick Sargent (the second actor to play Darren Stephens on Bewitched) and to the fact that in her real life, for a few years in the early 1990s, Carrie Fisher’s romantic partner was the Hollywood agent Bryan Lourd (father of Fisher’s only child, her daughter, Billie) before Lourd left her for a man.