Gilbert telling Annie that he loves her, finally resolving his unfinished business on Earth by falling in love, and then stepping through his Door into one of the show's rare portrayals of a good afterlife, smiling at the blue light.
Werewolf George backing away from the door in Season 1 finale and no longer trying to kill what's on the other side when it looks through the peep-hole, sniffs, and realises Nina is on the other side. Depending on your interpretation, it's sweet either because a bit of George remained to save the woman he loved, or because the werewolf doesn't want to harm the now infected Nina, its mate, which is still sort of cute.
Mitchell and Annie's reunion in season 3.
What George, Nina, McNair and Tom do when they're transformed with Mitchell and Annie in a cage in the room. After trying to kill Mitchell for a bit, McNair and Tom play-fight while George prowls around protecting a resting Nina.
Oddly enough George staking Mitchell is actually one, releasing his friend from his torment.
All the moreso when one considers Hal. He's spent five centuries cycling between unimaginable evil and the attempt to rid himself of it, with no guarantee that he'll ever stop. Mitchell, in hindsight, may well have been heading down the same path. And George saved him.
McNair's last letter to Tom and Tom's reaction is one.
More of a Fridge Brilliance thing as well, but if people who die go thru a door, and are ghosts for a moment or two, that means people who die together (car crashes, sinking ships, battles) would be able to go together: They wouldn't have to face it totally alone.
No longer just Fridge.
Sasha's final moments.
Sasha: It all falls away. The looks, the money. Lovers, Friends.
Annie: Not all your friends.
Sasha: No. Not all your friends. And the ones that are there at the end, they're the real ones, aren't they?
Annie: Yeah. Yeah.
Sasha: Thank you. For being my friend Annie...
Annie:(smiles) It was easy.
Leo and Pearl's departure qualifies as the wonderful heartwarming end to a 55 year old story about a Vampire, a Werewolf and a Ghost, which sounded much happier than the one we're watching.
Hal's prequel with Leo was especially touching- showing how a evil Vampire lamenting his darkness and a borderline suicidal Werewolf chained to a wall started a friendship that lasted over half a century.
Annie and Hal buying Tom his first birthday cake for his 21st birthday.
Tom handing himself over to the Old Ones with Eve as they promise to protect her, prepared to to live a terrible life of servitude out of his love for her.
Tom explaining to Hal why he is helping him kick his blood addiction
Tom: Because you're my best mate
Annie earning her happy ending in the finale of season 4, saving the world and seeing Nina, George and possibly Mitchell again.
Alex and Hal persuading Tom to return to the house instead of living in the woods, saying that he can fight against the beast inside him. Alex persuades him that she needs him to stay sane, Hal tells him that he can fight whatever darkness he has inside him, and both tell him outright that he is the best person they know, and certainly the best out of the three of them.
After being mean to Bobby due to left-over suspicion from Larry, Tom finally becomes great friends. They embrace when Tom reveals he managed to stop Bobby from killing anyone when transformed, Tom gets Bobby his first job, and Bobby gives Tom a massive Bear Hug when Tom buys him an answering machine from Ebay. The answering machine can play a tape Bobby hasn't heard since 1996, which holds his only recordings of his mother's voice.
Hal and Alex just smiling at each other when Hal survives Bobby's attack.
Fifteen years ago Rook found the only survivor of a massacre by vampires, a small girl. He puts on a plastic glove to touch her (in order to preserve evidence), having talked about his code of "no care, all responsibility" in his work. But seeing she's scared, he takes off his glove and takes her into his arms, promising her that he won't hurt her. He's a very ruthless man, but he shows his human side on rare occasions.
When Hatch offers the housemates their paradises all of them turn him down because they wouldn't get to be together.
The housemates turning human after killing the Devil, even if its not real. Particularly when Hal sees his reflection for the first time in centuries.