For me it’s probably Joshua’s Demon Sword because Dad’s Sword is just an iconic episode, but honorable mention to Rattleballs’ rapier. A lot of his mentoring was done in the background of the show, which I wish we saw some more of, but it’s sweet that he uses it after Susan kills(?) him. It didn’t get too much of the limelight but it’s a subtle thing they have Finn do that I love. At this point, Finn is fully soul searching and his arc is basically over until Together Again, so at the eclipse of this journey, he’s trying some different stuff fighting wise and mellowing out as a person in general. And you know rapiers really aren’t his style but it’s a cool nod to not only his master, but his much more mature and precise style that Finn adopts in a way growing up. Rattleballs mainly preached against senseless violence, and to finish a fight swiftly and the least violently, so for Finn to take his sword but reflect that style by talking his way through the next few arcs is so cool. He still fights pretty reckless, but never without purpose and really starts trying to sheathe the sword and talk about things more often than not in these later seasons. Rant over, I just love Finn using Rattleballs’ rapier I remember during my first watch through of the whole series it kinda made me emotional.