Completing the romance subplot with Aribeth in the original game leads to some pretty good moments.
In Shadows of Undrentide, ask Tymofarrar for Deekin's freedom and he will give you the stuffed doll for Deekin. Upon examining, there is a note from Tym to Deekin with the doll: "Search for your dreams, lad, and good luck. - T.".
At the start of the final battle in Hordes of the Underdark, Mephistopheles attempts to turn your henchmen to his side. There are several options you can use: a high Persuade score will turn them back, or you can use their True Names if you know them. If one is your love interest, you can use The Power of Love. But for Deekin, you don't need any of these, all you have to do is tell him you'll miss him. Aaaw. A little kobold chooses to side with you against the second most powerful devil in the Nine Hells, and this regardless of how you've treated him throughout the game. For that I love Deekin so very, very much.
Here's the quote:
M: "Come, join me. What has <Player's name> done but hold you back from your potential? I could make you a force that all dragons would bow down to in fear! All the world would know your name, Deekin, and fear it!"
D: "No! You may be bigger than the boss, but... but... DEEKIN NOT LIKES YOU!!"
M: "You dare defy me, reptile?!"
D: "That's right! Deekin defies you! Deekin stands by the boss... to the end!"
It's even more heartwarming if you use his True Name before the battle, which lets you tell him to believe in himself.
Of course, having a romantic relationship with Valen makes for a different kind of Crowning Moment of Heartwarming. Valen's always struggled with his tiefling blood. Mephistopheles offers him power, but he loves you enough to turn down the second most powerful devil in existence, who appears to be in a position to take over the entire mortal world.
The Sleeping Man's ending in Hordes if you wake him up and his true love is someone he can actually be with. The epilogue says he finally found her, and happy together the two spent the rest of their lives traveling the planes in an eternal honeymoon without a care.
Aribeth's recruitment in Hordes. The player can tell her to let Tyr decide if he will have her again after all she's done, rather than wallow in self-pity and doubt over it. Finding no fault in your words, Aribeth raises her sword and calls on Tyr to judge her accordingly with all her deeds good and ill laid bare before him. Tyr takes her back and transforms her into a Paladin once more.