The Man from U.N.C.L.E episode that starred both Shatner and Nimony in the same episode. But I'm sure most trekkies alreay know about that one. (its the episode that got me into TMFU)
As it happens, I'm currently renting that series from Netflix, and the Shatner/Nimoy episode, "The Project Strigas Affair," is the very next one I'm going to watch. I have seen it once before online, making it the only TMFU episode I've seen before the past couple of weeks, though I did see the '80s revival movie when it first aired.
Speaking of TMFU, I have been surprised by Jill Ireland's two appearances in the series so far, because with her shorter hair and the very different character she played, I could barely recognize her as the same person who played Leila Kalomi. I was also amused that her first episode, "The Quadripartite Affair," featured a heroic Harry Mudd and an evil Oscar Goldman. (I.e. Roger C. Carmel and Richard Anderson.)