Kuma's ending from Tekken Tag. It's probably the only time you'll ever see Heihachi as a kind, grandfatherly figure.
Surprising for what's usually a joke character, Paul's ending in Tekken 4. After winning the King of Iron Fist Tournament 4 and having everything he ever wanted, he realizes that it's not him after seeing a poster in Marshall Law's dojo. It then cuts to him starting up his motorcycle, he takes a longing sniff of gasoline, removes his rich shades, and mutters, "Ain't my style." He then throws them away, and drives into the sunrise.
Lars' ending in TTT2 is both this and Crowning Moment of Funny. It's about the first time in the series the Mishima actually get along. With the Kazama as well, for that matter.
Xiayou's TTT2 ending. She, Miharu, Alisa, and Panda are all at the amusement park, and Xiayou marvels that it's been a long time since it was so peaceful. A freak rainstorm causes them to take shelter, and Xiayou wonders about the meaning of life, and thinks they should come back in twenty years. Then, a rainbow comes out.
Baek's TTT2 ending. He has a flashback to the time he accidentally killed his father and breaks down, silently crying. Hwoarang comes along and sees him, and to take Baek's mind off of it, starts playing the lazy student, deliberately talking loudly about how he slept in so Baek will have something else to focus on other than his memories. Baek is visibly grateful for the distraction.
Forest Law's TTT2 ending as well. He and Paul got $100000000 and decided to spend it in Las Vegas, he lost all his money and understandably made Marshall mad (especially since he left without his dad's consent)... however, it turns out at the gates of Las Vegas, both of them encountered a crying family outside their burning home. On Forest's suggestion, both of them donated all their money so the family can buy a new house and cancelled their trip to Las Vegas. Yeah they did feel good afterwards.
Nina's Tekken 4 ending. She has been hired to assassinate Steve, but then makes the shocking discovery that, while she was in cryosleep, she was impregnated... with Steve. Later, she aims down her sniper rifle scope at Steve, prepares to pull the trigger... but she can't bring herself to kill her son. It's one of the few times that shows Nina does have a heart, after all.
Made even better when you see that in his ending, Nina actively guns down some men who were about to kill her son.
Jun in Tag 2 had become rather cold and, going by her Animal Motifs (ravens and crows) and quotes murderous who doesn't seem to get along with anyone. However there is a special tag ending with Jin, it might depend on which version is used or how they are customized as it's hard to get, but they'll begin to walk past each other before Jun (or Jin, depending on who finishes the fight) stops, turns and gives a genuinely warm smile.
Jack-6's ending in Tag 2, reminiscent of the ending to The Iron Giant. Just as an asteroid is falling towards Earth, Jack-6 takes flight towards the asteroid and starts hammering away at it, only to have minimal effect. It looks like Jack-6 is about to fly away from it, only to fly towards it full throttle and uses all his power to break the asteroid into bits and sacrifices himself in the process.
Jack-5's ending in Tekken 5. Jane installs the previous Jack's memories into him and boots him up. He suddenly flies into a rampage, almost kills her... and then he remembers her and picks her up. Jane's oldest friend is back.