The one that smells best on you. I have a cheap $50 bottle of Gucci Guilty sitting next to a $150 bottle of Tom Ford. Both smell great on me, but for different reasons.
Main things for picking out a scent:
1. Go to places like Nordstroms or high end stores if you are new to buying, the people working there know what they are doing.
2. If you don't know what you want, just tell the person what kind of natural smells you enjoy (Sweet, musky, clean, pine, etc...).
3. Don't try and smell everything, after four samples your nose is shot or burning out (and always smell coffee between samples).
4. Cologne that smells good on one person may not smell good for you, my body chemistry makes YSL smell like battery acid.
5. When you try cologne on, make sure you are clean and aren't wearing anything with strong smells. So if you own Axe body wash, it will ruin anything you put on.
6. Only try on two colognes at a time, have them spray it on your wrist in a small patch.
7. Cologne changes with time. It may smell good to start, but ten minutes later could be "meh". On the other hand, you may find it gets better with time.
8. Don't be afraid to walk away, if you didn't find anything you like then try again after you've washed off and gave your nose a rest for the day.
9. Don't buy the big bottles, even if you wear it every day, the small bottles will last a year. And by a year, you'll be tired of it and want to try something new.
10. If you can afford it, get a light easy going scent for work and a bolder one for the night out that can stand out over all the smells in a club/bar.
Most of all, just buy what you think smells good. If you enjoy it, then that's all.