Yamato helping to restore Bull to normal despite Ababa's brainwashing in episode 10 "The Bull Identity" thus returning him to his original self. All while battling him in the First Round of the Winner's Tournament at the same time.
Episode 13 "Hog Wild" had Yamato and Li preform a tag-team match. It was pretty cool to watch those two actually work together, and work together well.
Yamato and Enjyu's match in the first season finale.
Despite his Undying Loyalty, watching Joshua finally stand up to Cain and challenge him to a BDabattle in order to help him and protect Yamato and his friends.
Joshua: (points his BDaman at Yamato and his friends, then slowly turns to point it at Cain) Only you are my enemy now Cain. So I'm challenging you!
Cain: (looks on in shock, unable to say anything)
The final battle of the first series. And the ending where every single character comes to watch Yamato and Gray battle.