I've been planning for a while to start a thread about favorite mystery/detective series, but you beat me to it with this. In line with that concept:
I've decided that my favorite series of all is the Nero Wolfe books by Rex Stout. A little cozy, a little hardboiled, quirky as anything. It shouldn't work, but it does because of great characterizations, witty badinage and clever plotting. And, not inconsequentially, there are a ton of them and the quality is consistent throughout. The icing on the cake is the New York setting. I've been bingeing on them lately and the great thing is, by the time I get to the end, I can start all over!
I compare that to the equally witty and clever and characterized Hilary Tamar books by Sarah Caudwell, but there are only four of them.
I'll also mention the Judge Dee books by Robert van Gulik, clever and a look at an entirely different time and culture and entirely persuasive, as well.
I personally would like to hear about other people's favorite series, but if I'm hijacking the thread by making that request, I'll gladly delete it.