What fixture would give the best light in this high-ceiling room?

Non-trivial solution, but I would look at running a wire up the wall to the ceiling and over to the beam and either hang a chandelier from the middle of the beam, or install track lighting on the beam. You can use moulding along the beam to cover up a wire there so nobody will notice it (I did that for my living room which is now my dining room). If you do it right, you will only need to cut & patch two small sections of the wall/ceiling - one directly above the existing box and one next to the beam.