1. Sherlock Holmes
The world's greatest detective has appeared in comics from time to time over the years (though I still feel like it's cheating by including on here, but here we are).
The greatest detective since Sherlock.
3. Tim Drake
Is popularly considered to be Batman's equal in intellect (or close to it), and for good reason as demonstrated in the Red Robin series before it was canceled.
The world's greatest paranormal investigator.
5. Barbara Gordon
Specifically her time as Oracle proved that she is a very intelligent detective.
He was the only one who saw that something was not kosher about the Comedian's death. It was his investigation that uncovered what Ozymandias was really up to.
7. Vic Sage
I'm only really familiar with the version from Bruce Timm's Justice League Unlimited, but if that version was at all faithful to the character, then I'd say he definitely deserves to be on this list.
8. Martian Manhunter
Started out as a detective in his civilian identity. No clue if he still does that or not, but he was great at it.
9. John Constantine
Another very talented paranormal detective.
Almost always has the answers when it comes to investigating the occult.