I loved a lot of the art and cards over the years, but I'm a word person.
KYD Tadaka's "You run, I die." (comma splice not withstanding) always resonates with me.
As for art? Kachiko's art from letting in the Thunders (which they wisely centered on Time of the Void) is one of the single most impressive pieces of art in the game for evoking a pivotal moment and making people who don't know the game want to know . . . more . . . about the game. I know, because I've taught more than 140 people how to play the RPG, and I always use that art as a selling point to convince them to play. I have never known it not to work.
Interestingly, most of my favorite characters had rather crappy art on their cards.
Remembering characters like Kachiko really shows how flat the storyline became. She really was such a powerful personality.
I think that depends of time when someone entered a game. Everything before Samurai was a myth for me so that's why Chagatai and Khan's March are more closer and vibrant for me than Seven Thunders, Clan Wars etc because i joined game then just before Lotus end.
Chagatai was a really good story arc too. I was more referring to Ivory edition. There were loads of Iconic characters around after Spirit wars too.