The first thing to consider is body chemistry. Various fragrances will react differently with your own body and affect the scent.
Apply a small sample of whatever cologne you are consdiering on the inside of your wrist and wait a few moments. Give it a smell, and get a second opinion, if possible (as long as it isn't some over-achieving sales clerk). If you like the scent, then you might want to go ahead and purchase the cologne.
I believe I read somewhere that the chemicals in colognes start to break down (or something like that) after six months or so, and that this can change the scent, so you might want to limit yourself to 2 or 3 fragrances that you use in some rotation. I have some Intuition by Estee Lauder that was purchased back in mid 2005, and the scent has become very weak over time.
As for my two scents (pun intended), I like Burberry Brit for Men as a daytime fragrance, and Dolce & Gabbana Pour Homme for the night.
The first is light and fresh with hints of citrus, while the second is heavier and has more of a musk.