For me it's landscapes. I was pondering this over the weekend. I shoot mostly wildlife. It dawned on me that I've concentrated on wildlife for two reasons. First, it gets me out in nature which is why I got into photography to start with. Second, it's challenging from the standpoint that one has to "hunt" the subject first to even get the opportunity to take the photos. Once that's done it's then just the technical process of capturing the images. You have no control over what the animal does so you kind of get what you get and if you're lucky the animal does something interesting. Likewise you have no control over the lighting conditions etc. when you make the encounter. Granted you can take best advantage of what's available but basically it is what it is.
Landscapes, on the other hand, just sit there. You have to think about it, move around for an interesting perspective, etc. Manipulate what's in the frame and what's' not. Position elements within the frame, blah, blah, blah. I've read umpteen things on how to do it well but when I'm there looking at a scene none of that knowledge seems to make itself available to me. After the fact I'm always thinking that I should have done this or that.
I was just wondering whether others have the same problem...