The legendary final few moments of the series finale, where the cast engages in a group hug for the ages. There's a reason that modern TV writers look to "The Last Show" as the pinnacle for closing out a series' run.
Another one earlier in the episode comes from Lou, who clears out the news room's petty cash box to fly Phyllis and Rhoda in from San Francisco and New York respectively to cheer Mary up.
Lou realizing that he really cares for Ted when the latter decides to take a new job as a game show host in Season 6's "Ted's Moment of Glory". When Ted is saying his final goodbyes to everyone at the station, he comes up to Lou, who initially is unable to say anything but one word:
After Ted reveals to everyone that Murray actually penned a successful article that the former was supposed to write in Season 7's "Murray Ghosts for Ted", Murray drops by his dressing room to thank him personally. With it airing so close to the end of the series' run, it's particularly gratifying to see how much the two could care for each other.
Murray: For a long time now, I made you the butt of my jokes because you were there and you happened to be a preposterous figure. But what you did today was classy. You're a fine man, Ted Baxter. It's an honor to know you.
In what could be seen eerily as a parallel to Gavin MacLeod being the only MTM cast member never to receive an Emmy nomination, Season 7's "Murray Can't Lose" chronicles some of Murray's frustrations with never managing to receive a win at the Teddy's Awards for his work, while the rest of his colleagues are often acknowledged. In the episode itself, Lou hears from an inside source that Murray is apparently a lock to win that year, and Murray becomes excited by the prospect of some recognition. However, Murray ends up losing, left out once again (while Mary, Lou, Sue Ann and Ted all managed to win that very night). In the end, Murray's wife tells him to read out what would have been his acceptance speech, and it results in a Crowning Moment of Heartwarming when everyone finds out that the speech was going to highlight the depths to which he loved all of his friends.
A subtler example, perhaps, but the theme song lyrics changing from season 1's "You might just make it after all" to season 2's "You're gonna make it after all".