I like the Penn balls that I buy by the case at Costco. I also like the Wilson US Open balls.
I absolutely hate the Wilson Titanium balls - they feel very hard and unyielding to me, like the underlying rubber (or whatever the material is) is stiffer than other balls. I don't even like to warm up with them and I would gladly open my can myself rather than play with an opponent's Wilson Titaniums.
For a while, my club was selling Prince balls, and I liked the way the felt and thought they were a little more visible, too. But a local shop owner told me that while people liked playing with them, they had a relatively high incidence of pressurization failure in the can, leaving the balls DOA.