Noble
and I think for quite a while I´ll continue playing this level most of the time.
Maybe I could beat the AI, too, at higher levels,
but, as dabrovo mentioned,
the noble level seems to be the most balanced
regarding the bonuses the AI and you get
and I play the game just for fun,
not because I want to prove that I can beat the AI
even if it gets bonuses.
Maybe I´ll think different some time in the near or far future,
but at the moment being at noble level
and just varying things like random map scripts, map size
and/or number of civs is enough to keep me interested
(after all I also don´t have too much time to play civ,
which means that games on large/huge maps
can sometimes last a week or longer

)