Definitely fog is easier. We get both around here, once in a while the kind of fog where you REALLY can't see even a few feet in front of you and have to just find the line for the road and follow it (and hope all the other cars were smart enough to turn on their lights so you can see them before you hit them) as you move at 10 mph.
However snow has white out conditions too, while also making slippery roads and that would also mean windy conditions. This is much worse.
The times we get the torrential downpours that you can't see through are also worse. My area has a few intersections that also don't drain right so they become lakes and cars stall and end up stuck in them.
I think my list of worst driving conditions would be:
1) freezing rain - This makes a sheet of ice over everything... very pretty almost like those places that have flowers etc that are encased in glass... but its literally an ice skating rink, not even safe to try to walk on never mind drive.
2) Sleet/slush - These are items that come at about the same point of freezing rain so there is a possibility of getting that instead.
3) Snow with white out conditions
4) The first nice day in spring where everyone decides the roads are finally good so they are going to dive 20 mph faster then they should (we get more accidents on this day then most of the days with snow...)
5) Snow without the wind to make white out conditions
6) Extremely heavy rain
7) Thick Fog
