"OBCR of A Little Night Music for sure; the revival is splendid as well, and, fun fact: the woman who originated the role of Anne (Victoria Mallory), her daughter (Ramona Mallory) played the same role in the revival! A very interesting juxtaposition."
A fun addition to that fun fact is that Victoria Mallory is married to Mark Lambert, who played Henrik in that original cast (he's also Ramona's father, she changed her name for the stage).
My favorite Sondheim recordings are the above mentioned ALM, as well as Gypsy, Merrily We Roll Along, Company, Side By Side By Sondheim and Sondheim: A Musical Tribute (I love all his stuff but those are probably my favorites).
"Contentment, it seems, simply happens. It appears accompanied by no bravos and no tears."