I voted Office/Desk Area, but would like to extend that to working areas in general. Be it a library with desks and computers, an artist's studio, a music or archaeology room - to me, this is the kind of room that gives me the most freedom and the least annoyances. Second would be bathrooms and laundry rooms (which I often put together), they're quick and easy to do.
Everything else I hate because of how hard it is to find matching stuff, and not being able to colour different parts of single objects separately severely cuts down the options, especially bed frames and sheets rarely come in the combination I need. If not for that, I would love decorating bedrooms as well because of the aforementioned personalities you can express through them.
