I don't want to say too much, because often the cases may start as very regular crime investigations such as murders/kidnappings, theft etc. then a villian may (or may not) end up involved, but I'll say straight up say the Joker as he appears on the front of book 2, and one other that you mentioned. The most interesting thing about the book is it's very much focused the titular GCPD. Examining crime scenes, evidence, visiting the morgue, arguing about who likes who's partner etc., it really captures the feel of the wire, or even something like the shield. Villians and batman do show up, but their appearances can be fleeting despite them being the *main* threat to the city; this is what the police do to cope with the fact the villians and the bat are running around the city leaving them entirely out of the loop. What are they doing while Joker is leaving traps around the city but nowhere to be seen and Batman is down in the sewers following a lead? It's fascinating stuff from Bru and Rucka. I would love for them to reunite for a completely straight police book but Gotham Central is fantastic in its own right.