Hello! Sorry, I saw this at the wrong point, and then promptly forgot it was in my inbox, lol. I hope you're still writing!
So, yeah, in terms of scenes I go back to: that often depends on the emotion I'm trying to track, or the particular relationship. But in terms of ones where I feel like he's really got some screen time that's worth watching:
Ep 1: the funeral scene/fight with Luther. Bitter but righteously angry Diego at his finest (with the sweet little twist of his concern for Grace at the end).
Ep 3 (Extra Ordinary): the flashback to Grace helping him with his stammer as a child, and then the two scenes with her as an adult - with Diego trying to persuade her to leave, and then finally switching her off (a heartbreaking little sequence!).
Ep 4 (Man on the Moon): The conversation with Luther in the library, before they find Five - Diego connecting with his brother and trying to break through Luther's blind faith in Reggie.
Ep 8 (I Heard A Rumor): The little scene with Beaman - example of how good Diego is at making friends, despite his prickly exterior, and Beaman sees the good in him that he saw in Eudora.
Season 2 (aka the best Diego season):
Ep 1: Him (and Lila!) in the asylum, talking to Dr Moncton, doing arts and crafts, and his conversation/confrontation with Five. And then his escape with Lila - man, this is SUCH a good Diego ep.
Ep 2 (The Frankel Footage): His plan to chop off Oswald's finger, lol, him and Five arguing about whether Reggie is up to something, and his lovely little reassurance of Lila in the darkroom. And then him and Five breaking into Reggie's place and talking about their dad some more, and his fight with Reggie and ultimate inability to kill the mf.
Ep 3 (The Swedish Job): Him and Lila talking about her parents, and...all the rest of that scene. <3
Ep 4 (The Majestic 12): Him and Lila being domestic and cosy in the morning, their dance at the consulate, his conversation with rl Grace (his sheer horror at the thought of Reggie and Grace dating, lol!), the fight with the Swedes.
Ep 5 (Valhalla): Diego's confrontation with Lila. His easy forgiveness of Vanya, and attempt to tell his family about his girl troubles. His "Everyone changes the world" speech.
Ep 6 (A Light Supper): His gleeful little "Team Zero" plan. Being destroyed by Reggie.
Ep 7 (Oga for Oga): Him and Luther phoning poor Olga. Him hugging Ben/Klaus. Taking the time to bury Elliott, and falling for Lila's transparent plot.
Ep 8 (The Seven Stages): Meeting the mother-in-law (instant enemy). Meeting Herb (instant friend). Being the only one to greet Ray like a normal person.
Ep 9 (743): Desperately trying, and failing, to save JFK.
Ep 10 (The End of Something): Him and Five confronting the Handler and Lila, and then saving one another from the Commission army. Talking Lila around, and stopping Luther from stopping her. His brief little moment with Vanya at the end, as they deal with leaving/getting left by their loved one.
I'm gonna come back to s3 bc I have an evening to live, lol. Hope this is helpful!