I personally think 7 or Serpent Isle as the plot is easily great and the graphics ok too.
Out of all of the Ultima games which I have played, my favourite was Ultima VIII, even though I have played little of this series as a whole.
Ultima 9, it was a beautiful “ending” to the saga.
Hey, you didn’t include Akalabeth. It’s the only Ultima game I’ve ever finished. 