I love TWOTW, and You Are Cordially Invited, ITPM. Emissary and WYLB are very nice bookends to the entire series. More than I can name really.
Are you ever in a mood to look at particular episodes, like the Feringi episodes, or the arc beginning with A Call To Arms through Sacrifice of Angels? There are many ways to appreciate this series. In The Cards or, Treachery, Faith, and the Great River, and Take Me Out To The Holosuite are very amusing and lighthearted. FBTS, Waltz and ITPM are somewhat unsettling, tense, but riveting.
I've never been able to name just ONE!