Master, and I wouldn't have it any other way.
During my first playthrough with a 2H Nord warrior, I started on adept, and I had only gotten to level 10 when combat started to feel like child's play. I upped the difficulty to expert, then to master and eventually incorporated a custom difficulty mod which gave me 0.4x damage output and my enemies 3x damage. Even that felt a touch too easy at times, excepting spellcasters, which could sometimes one-shot me. I think I managed to create an overpowered character through smithing and enchanting, though I never exploited the infamous alchemy-smithing-enchanting loop.
During my second playthrough as a sneaky Khajiit, I started the game on expert and moved up to master at around level 20. I'm currently @ level 30, and the difficulty is spot on: combat is challenging but not frustrating. I die quite often if I don't make smart tactical decisions before instigating combat, and I sometimes find myself running away from overpowering foes. That has forced me to develop strategies, think ahead, summon help if needed and pick my battles carefully. This
never happened during my first playthrough, I was totally self-sufficient and rarely needed the assistance of followers or summoned allies. With my Khajiit I try to backstab and pick off enemies stealthily with a bow, and if I am forced to engage in direct melee combat, I have to make sure I have an escape route available. Powerful melee enemies are my mortal bane if they manage to corner me; I often get decapitated by 2H-wielding Bandit Chiefs and the like, and a single snap of a dragon's jaws can be my undoing.