"Samaritan": Matt's awkward yet utterly endearing efforts to support Alesha after she's raped—asking her how she is, telling her "I think about what happened to you a lot", promising her that he's here for her, then hurriedly assuring her that he's not trying to make advances, to which Alesha teasingly responded "why not?", causing Matt to stumble even more as he admitted, "I'm a bit crappy at this", and Alesha gently telling him that he was doing fine.
Any bit with Matt or Ronnie trying to take care of the other.
The end of "Honor Bound": "I'm so sorry Ron. I can't make it better. But...chips? On me?"
That and his frantic reaction at the beginning of the episode when he thought Ronnie may have been shot.
"Confession", full stop: from Ronnie keeping hold of Matty at the crime scene, to the Guv telling hm to watch over Matt, to the end scene in the church and "I'll be outside when you're finished."
Ronnie's line, "God forbid Matty here got himself shot, I'd be out there straightaway trying to find out who did it and string him up myself." It became quite Harsher in Hindsight given Matt's eventual fate, but at the time, it was genuinely touching to see just how fond of Matt Ronnie had become.
The bit in "Help" after Ronnie gets held at gunpoint. Especially the subversion that it's *Matt*, and not Ronnie, who's most shaken by the whole thing, and is on the receiving end of a nice cuppa.
"You ain't getting rid of me that easy, sunshine."
And finally, in "Survivor's Guilt"- so many moments were heartwarming even through the tears of Ronnie and the audience but especially this:
"The police officer, who died? He was like...like *my* son."
And also, “When someone you love dies, the hardest thing is to be left behind. You’d do anything to bring them back. You’d take their place.”
In a deleted scene from the ending, after Sam takes the glass of whiskey away from Ronnie, he gets up to leave the bar with a pet carrier containing Matt's cat—now his cat, essentially symbolizing that even though he'll move on with his life, he'll never forget him.
In "Tremors", the mention that Ronnie's grandson had been named Matthew.
In "Hard Stop", his memorializing of Wes plays over scenes of him entering a convenience store and buying a bottle of vodka. As we hear the words, "You've never let me down before. . .and I won't let you down now", he leaves the bottle at the cash register and walks out of the store.