That is a very subjective thing. Everyone is going to have a different opinion as to what they like. Many people, myself included, have tried different styles and gradually narrowed down what works best for the individual in question.
My personal preference, for the moment, are Heinie "Slant Pro's", all black, with CGR race cut. Most shooters, from what I've seen, generally, come to the realization that, a wider rear sight window and/or narrower front post allow you to get on target faster, and with more precision. There is something about having more visible light around the front post, that helps me find it quicker.
Some people prefer an all black sight picture. Some, a black rear and fiber optic front. Or any number of other combinations of metal, fiber optics and/or tritium (night sights).
It depends also, on what your goals are. If you want to shoot competition or just plink at the range that may affect your decision. If you decide you want to be a "gamer", your going to need sights that allow you to get on target as fast as possible. Again, this is subjective, based on personal preference, skill level, eyesight deficiency and other factors.
Ecmills did an excellent evaluation of some popular competition/target sights. Follow this link...