Monarch. I feel very comfortable and confident playing monarch, but I have a feeling that my approach is too tailored for a particular set of conditions - map configuration I'm used to, civ traits I'm used to, UUs I'm used to, etc. I picked up the November GOTM, which was Monarch but otherwise lots of game conditions I'm not accustomed to, and I got, well, spanked.
So rather than moving up a level to increase my challenge, I'm going to try randomizing more things, including what civ I am playing and what the map's like and such.
As an aside, I'll comment that it's neat how the result this poll is looking like such a clean normal distribution, peaked between Regent and Monarch.