If the criteria for an "underratted" episode being that it was not included in "The Best of TMS" releases (and the two I listed were not), then the first season has a lot of underrated episodes. Paul Williams, Avery Scrieber, Kaye Ballard, Ethel Merman, Connie Stevens, Valerie Harper... But otherwise I really don't consider most of those to be underrated (well, maybe Valerie Harper and Kaye Ballard).
I also think of the Joel Grey, Peter Ustinov, Lena Horne, and Phyllis Diller episodes as underrated. The Joel Grey episode is the first one after the first two pilots. The first two episodes had material cut and replaced before they aired to make them stronger episodes, but the Joel Grey one didn't, but I like a lot the rather "raw" early feel of this episode. All of the numbers are great, the Sherlock Holmes sketch is very funny, but the backstage plot is kind of weak, and there is a bit of "early installment weirdness" regarding Miss Piggy, mainly that she doesn't have much personality and her roles could have easily been given to anybody (the same can be said for most of Miss Piggy's scenes in the first 6 or so episodes).