For the fence we built last year, we used a top rail of the 5" Centraur HTP Vinyl flex fence along with 4 strands of Centaur vinyl coated high tensile mounted on 5" pressure treated posts. We felt this was the best combination of price, visibility, and safety for our needs. We intend to have boarders so we went more conservative on the safety aspect. If I had the cash, I would have put in 4 strands of 5" flex fence. In my opinion, this is probably the safest way to go - if a horse hits it, they will just bounce off.
In central DE, the the vinyl coated high tensile is becoming very popular. In Pensylvania horse country (where the DuPont, Cambel Soup, Smith Kline Beechem Heirs live) the most popular fence is diamond mesh with a board on top for visibility / rigidity. Since money is no object to these people, my guess this is one of the safest set up.
That being said, there is no perfect horse fence - all types have their pro's and con's in regards to price, safety, maintenance, etc. We had a horse a couple of months ago get its leg wrapped around one of the strands of vinyl coated high tensile. /forums/images/graemlins/shocked.gif Don't ask me how she did it - the strands on our fence practically sing in the wind from the tension. Luckily she didn't struggle much and we had to cut the wire to get her out. She was sore for a couple days afterward but no worse for wear. The vinyl coated wire prevented it from digging into her. If it was the bare high tensile wire, she would have been cut up bad.