My favorite genres of mystery are the police procedural (think Elizabeth George) and cozies. But I've found that I'm particularly partial to the amateur sleuth cozies.
There are many aspects that draw me in. For one thing, solving the case is similar to working out a puzzle. The sleuth is getting the clues at the same time the reader is. How, or if, we put them together is up to us.
For another, I like the way that the sleuth is working on a different level than the police. Instead of the forensic evidence in a police procedural, books featuring amateur detectives frequently employ different information-gathering techniques. These may include listening to local gossip, forging relationships with police and other people in the know, and being, in general, a busybody. The amateur uses their natural talents and "little gray cells" (in the case of Hercule Poirot) to solve the case instead of relying on DNA droppings.
The settings for the sleuth novels are another draw. The reader is frequently treated to a visit to a small town or village. We are introduced to a limited number of suspects, whom we get to know well.
Amateurs make mistakes that lead us on red herrings. Or they believe witnesses and gossips that take us on wild goose chases. Sometimes, realizing we've hit a wall heightens the urgency. We can sympathize with the sleuth, since we're in the same boat--we haven't solved the mystery, either.
Amateurs aren't loaded down with weapons in their battle with the villain---usually their only self-defense is a sharp tongue and a quick mind.
Sleuths are smart, connected, motivated, and human. It's fun to follow along as they work out the puzzle and bring a sense of normalcy to their quiet corner of the world.