Seinfeld is the last show you would ever expect a heartwarming moment from, and in accordance with that perception, nearly all the moments are only heartwarming because they are tinged with meta, but there are a few worth mentioning:
Most obviously, the montage of still frames, behind-the-scenes footage, bloopers and classic scenes at the end of the penultimate episode, "The Clip Show", set to "Good Riddance (Time of Your Life)" by Green Day. I dare anyone who watched Seinfeld during its original run to not break out in goosebumps and feel unable to stop smiling. Doubles as a Tear Jerker, especially during the line "We hope you had the time of your life", because it was so true. The '90s were the time of most presently-living people's lives, and Seinfeld was the end-all-be-all of their entertainment.
In the canon of the show, the scene in "The Pilot, Part 2" (not to be confused with the actual pilot of "Seinfeld") where the actor playing "George" (in "Jerry", the show-within-a-show) comes running up to Jerry right before filming, freaking out and babbling that he can't do it, prompting Jerry to throw his hands up laughing, saying that he's perfect — he's acting just like George.
In "The Deal", Kramer's card for Elaine: "Think where man's glory most begins and ends/And say my glory was I had such friends." - William Butler Yeats
The viewers witnessing, by implication, how "Seinfeld" was created — George trying to convince Jerry that a show about nothing would be a good idea. Jerry's scoffing response: "Yeah...sure...that'll work." If only he knew...
The scene at the end of "The Trip Part 2" when Jerry tosses Kramer his apartment keys, the cause of the argument that led to Kramer moving all the way to L.A.
An episode where Elaine returned from a trip (the actress' return from maternity leave), leading to a genuinely enthusiastic few minutes where she hugged all three of the guys.
There are very few moments where what could be argued as love is shown. There are well wishes, appreciations, compliments, condolences, concerns and congratulations shared between the main four, but only once is there a hint of actual love: At the end of "The Deal", Jerry and Elaine admit that their deal is breaking their friendship apart. They can not simply have "that" (sex) without consequences and Elaine can't go back to just "this" (friendship). She wants "the other", a true romantic relationship. They start one. Both Jerry Seinfeld and Larry David said that this is the only moment where there is real true emotion between the characters. (Since this is Seinfeld, everything was back to normal by the next episode).
Another meta one: the reason why "The Raincoats" had so many Schindler's List references? It's because of Jerry (real life Jerry, that is) finding out that when making the movie, Steven Spielberg got so depressed he would watch episodes of Seinfeld to help cheer himself up.