First off, you are going to have a very hard time finding natural colognes because they almost don't exist anymore. Snythetics can be just as good and even better than natural ingredients. If you have worries about cancer than you should lock yourself in a room a sit there for the rest of your life because studies are saying that almost eveything has some effect on cancer. No sense in worrying about that small bit carcenogenic white musk, IMHO.
Sorry if that was too forward. I'll keep in on topic by saying that Vikto and Rolf, and YSL use natural and synthetic ingredients. There usually is a mix between the two based on the scent each yeilds.
I would deffinately think about Live Jazz, M7, M7 Fresh, and Rive Gauche. Those are very popular for a reason. Read the reviews and you'll find out what each on smells like. I own 2 of those 4 and have decants of the other 2. Good stuff. Viktor and Rolf came out with somehting new about a year ago around the same time YSL release L'Homme, which is also a good scent. I can't remember what it is called but it was decent.
See if you can get some samples somewhere because it'll save you money if you end up buying something you don't like.
Let me know if you have any more questions about synthetic vs. natural ingredients.
PS- I think my post was longer...