Young Justice season 3, episode 20, "Stronger Than Superman" had a sincere moment between Superman and Superboy. Conner Kent, the Superman/Lex Luthor clone, is getting married and has picked Clark Kent to be his best man.
Young Justice: Outsiders' premiere, "Princes All", had Conner Kent finally propose to M'gann, Miss Martian. While things mostly remained quiet on this front, episode 20, "Stronger Than Superman", brought things to the next level with Conner asking his brother Clark to be the best man at his wedding.
To much amusement, this sequence happens right in the middle of an Apocalyptic battle where the heroes are fighting Darkseid's Parademons, all the while convincing Metron to come back to Earth to save a dying Cyborg. Despite that, Clark is immediately honored to be asked and is even bemused that his brother is getting married in the first place. While the sequence itself is short, it's incredibly weighty when considering the history of the two characters.
Conner and Clark have seen a lot of growth in their relationship since Young Justice season 1. What started out as trepidatious, primarily on Superman's side out of discomfort from interacting with his clone, has now developed to the point where the two see each other like brothers and have a strong trust in the other. The sequence of the two interacting highlights how the gap between them has closed. The angry, destructive nature of Superboy from past seasons has given way to a calmer, more hopeful approach like his big brother. Likewise, Superman has loosened up in Young Justice: Outsiders and is seen using phrases and mannerisms of his younger brother's team.
From a show perspective also, Conner Kent is no longer in the shadow of Superman but truly feels like his own character. This goes to the credit of Nolan North, who voices both Superboy and Superman, yet makes each of them distinct. With Young Justice carefully developing these twin sons of Krypton, it gives extra weight to Conner asking Clark to be his best man.
While the moment is short, Conner meeting his older twin brother and asking him to be his best man exemplifies one of Young Justice: Outsiders biggest strengths. Virtually every Superman in recent media has been alone, with great focus on that resulting loneliness. In Young Justice, he has a brother that he truly feels close with, evident by the hug they opted for in the middle of an apocalyptic battle. It does seem like the wedding between Conner and M'gann is being saved for the Young Justice: Outsider season finale, and it will be great to see Clark Kent celebrating alongside them.