I'm definitely a Columbo fan. Monk just gives me a headache and Sherlock Holmes is an addict so how reliable would he be in real life? XD Batman has to come in second place and then for third it's Magnum, PI. What can I say, I'm a female! An honorable mention goes to Paul Drake, Perry Mason's PI.
My parents love detective stories and they have a pretty big collection of them (and other books too). I haven't read a lot of them so I'm no expert in this field, but I think Hercule Poirot is the best. I love it how he uses mostly psychology to solve the crimes. He is easily the best one of Agatha Christie's characters.