Saitoh during his fight with Kenshin in episode 30. The continuation of this fight that was never decided in Kyoto has taken them both back to who they were during the Bakumatsu, and the intensity of things can be seen here on Saitoh's face.

The episodes at the beginning of the Kyoto arc are probably my favorite part of the series. As for Saitoh himself, he's definitely not a nice person, but he does follow his own code of justice, for which I have to admire him. And by the end of things, I think he has developed a sneaking respect for the Kenshin-gumi, even Sano the "aho". :-)