L (also known as: Hideki Ryuga, Ryuzaki, Eraldo Coil, and Deneuve, at times when he's not going by a single letter) is the (fictional) world's greatest detective - constantly at odds with Light Yagami, aka a mass murdering genius megalomaniac known as "Kira." While he looks like an antisocial weirdo who eats wayyy too much candy, L's actually the most brilliant person alive - constantly analyzing and deducing things, and catches onto Light's out-of-control murder spree pretty quickly, although before he can do anything about it...well, don't wanna spoil anything here (even a several years old manga), so just go check it out for yourself.
You'd think by the fifth or sixth elderly amusement park owner disguised as a ghost was unmasked, the Scooby Gang would start to realize that most of the ghouls they investigated were just crazy people using trickery to chase off this vanful of weirdos and their giant talking dog. Actually, why didn't they investigate THAT? The fact that they had a dog that could talk? Answer: because the entire series was just an extended LSD trip by Shaggy. Duh.
Rorschach solved a lot of mysteries - who was behind the murder of costumed heroes, what was the secret behind Dr. Manhattan's supposed cancer victims, who was locked up in here with whom, etc. But his greatest achievement was (probably) solving the mystery of how to ruin world peace. Pretty impressive for a guy who calls blood "human bean juice."
Sherlock Holmes is basically shorthand for "brilliant detective" at this point. He's one of the most frequently adapted characters in all of fiction, and for good reason: he's a prettttty good detective (and also never shits, according to something my brother used to say to me a lot). He's a mostly emotionless, calculating, analytical, deduction-obsessed supergenius who solves crimes out of no sense of justice, but more or less to prove how brilliant he is. Practically every fictional detective has drawn upon Sherlock Holmes as an influence since. Modern audiences have THREE separate Sherlock Holmes being made - CBS's Elementary, BBC's Sherlock, and the Sherlock Holmes Shirtless Karate movies. But there's apparently one detective even greater than Sherlock. It's...
"The World's Greatest Detective" is appropriately the world's greatest detective. He solves his crimes through a variety of ways: headbutting, using his cave's supercomputer, punching, kicking, dangling thugs off of roofs, punching more, and occasionally throwing things. Well, usually at least. Sometimes he actually does do detective work - trying to determine the true identity of the Holiday Killer, the identity of Hush, and...well, there's more, but even the times he did do detective work, it was a lot of punching and gadgets solving things for him. Still, the results speak for themselves - he's put away more criminals than pretty much any other detective in history. Sure, most of them escape regularly, but that's really more of an Arkham Asylum security issue.