Kyoto arc, Rurouni Kenshin. Kenshin, in a lot of ways, was extremely childish, like when he pulls out a radish from his robe, or the fact that kaoru is
so goddamn clueless. The everyday happenings in Rurouni kenshin are arguably a lot sillier than those in say, bleach or naruto. The kyoto arc, (rewatch it and see) was
dark, and suddenly the show becomes very very serious. From the first fight with saito to the last fight with shishio, the arc is packed with great characters (anji, saito, soujiro, shishio, yumi). I also love the parallels between kenshin and shishio, as well as kenshin and sojiro.
Most importantly, it was as much a philosophical battle throughout as it was a battle of strength, much more than say, bleach (ichigo has got to be the most bland shounen hero ever), as well as naruto (while the philosophical question posed by naruto is interesting and has a lot to chew on, i.e. how do we stop the cycle of hatred, i didn't connect with the villains as much, especially considering the kyoto arc is only 30 odd episodes). On the other hand, both sojiro and shishio were great. shishio himself was a colossal character, remaining his evil dominating best until his end.
Next, the powerups (or powerup) was awesome and was accompanied by serious character development (both kenshins and sanosukes training). Not like bleach where, bam, random traning, ambiguous message, powerup! Apart from that we got to see a much more mature and complex side of kenshin.
Finally the lines.. some of the lines were just fantastic!