Kyou Kara Ore wa is what I would consider one of the classics of gag manga. How old is it? It was serialised in the 1980s and despite that, its brand of humor is still very relevant today.
What is KKOW about? It's the story of 2 wannabe delinquents, Mitsuhashi and Shinji, who decide to change their image and go from average normal boys to being utter badasses. In other words, fake it till you make it baby.
Despite all this, it works because somehow those two manage to overcome all their obstacles through a surprisingly high potential for combat as well as deep bonds of friendship. Kind of. Shinji being the stereotypical good guy is always the patsy and straight man, while Mitsuhashi; being described as more twisted than a pretzel, is the evil genius behind everything. Half of the humor comes from reading about Mitsuhashi's evil over-the-top actions in fact. In modern internet lingo, Mitsuhashi is the ultimate troll and yet, he's the type of person who will always defend the people closest to him. Which makes him a much more interesting character than the usual wussy spineless rejects we get from shounen manga no.
For an example of just how 'trollish' or cunning Mitsuhashi is, when he was first confronted by a much larger gang than him, he pretended to give in while Shinji fought back. Over the course of a week, the gang bullied Shinji without noticing that their numbers were decreasing. When they fought out Mitsuhashi had secretly been waylaying them on the way back home, it was too late because they were too few to fight back.
Overall the story spans the course of 3 years, going from their first year in Junior High to their graduation. Were there unfunny parts? No, not really. I think this is one of the few series where most of the manga is genuinely funny. Kind of like the original GTO. The whole series was translated into english a few years back and if you have a chance, I highly recommend reading it. Just reading the first few chapters is enough to get you hooked. Source: happened to me.