Chin Ho finally marrying Malia in "Alaheo Pauʻole". A Wedding Day is heartwarming anyway, but when you consider what Chin has been through and the reasons why he ended his first engagement to Malia, it's pretty clear that Chin has earned his happy ending.
The ending of "Kahu". Can be a CMOA too.
Steve shows up in his Navy dress blues in "Hana I Wa 'la" as a character witness for Danny in his custody battle to keep Grace with him in Hawai'i. Even though his touching speech on why Danny is a great father is interrupted by a Mystery of the Week phone call, it's still rather sweet of him. It's even a deciding factor in Danny winning the custody case. The whole ordeal shows how far their friendship has come since Season 1.
In "Ma'ema'e", Steve and Danny discover provocative pictures of a young girl in a suspect's home. They had been told earlier that the suspect was like a second father to the teenager. Danny's reaction is expected, but Steve proves to be just as infuriated as his partner about the possibility of anything like that happening to Grace.
Danny: I do not think these were taken by a second father... If I found somebody taking pictures of Grace like this, I'd go to their house and I'd kill them.
The footlong rant Steve provokes out of Danny by throwing the word "classified" around when he resurfaces after his time away looking for Shelburn. He keeps smiling throughout the whole thing and keeps prodding his partner until he runs out of time. He clearly enjoys having that bickering back and makes sure they both have their share before he had to board that plane.
Also, earlier in the episode when he calls Danny he hesitates for a moment because he know his partner will blow up on him, which he promptly did. Because he was worried rather than actually angry.
Danny using Steve's vocabulary on accident and Steve then saying he needs to get back right away. It's their subtle way of saying "I missed you". Aww!
Just about anything with Max and Sabrina:
Their first date is ruined when she gets shot during a bank robbery. So, he makes a vending machine menu for her and they have their date in the hospital while she recovers.
In the Season 4 finale, after Grover is forced into early retirement from HPD after saving his daughter. Steve and the rest of the team invite him to join Five-0.
The 100th episode "Ina Paha" starts out like the Pilot, except this time, Detective Danny Williams save Steve's father in time. It was nice to watch until we learn it was only part of Steve's hallucination.
But it is also sweet because in Steve's mind Danny could have saved his father and make Hesse talk. It shows how much he values his partner and how highly he thinks of him.