This is a great question! It made me start pacing around the room as I thought about it. I have a couple different kinds of favorites, but I think they're all in the general category of tileset. That's not to say I don't enjoy working on battlebacks and battlers (lots of fun) or parallaxes (fun challenge and an oft-overlooked category of resource) but tilesets are my favorite for a couple reasons. First, people are looking for tilesets, and it's always good to be able to provide people with something they want. Second, there's a lot of flexibility in tile art and while it's hard to compare in an apples to apples sense, I feel like there's just more freedom to do... well, whatever you want, when making a tileset.
I think I can narrow it down even further. There are kinds of tiles that are especially fun to work on. Furniture can be a real pain, but it feels really good to hammer out a couch, ornate chair, cabinets, and the like. So furniture (which can range from contemporary/normal kinds of stuff all the way to crypt-beds in a vampire abode) is up there. But is that my absolute favorite thing to work on in tiles? Not quite.
So here it is, my favorite kind of tileart to work on. I'm not sure what the term is, but the general idea is bizarre kluge-objects that presumably perform some kind of arcane, weird, or transcendental function. Stuff like this:
My attachment to this kind of thing likely comes from stories I've read as well as older video/computer game art.