The worst I've ever seen was in early 1990 in Brisbane.
Was flying a Citation 2 down from Cairns to Brissy and back and was sitting on the ground in Brissy at about 1am I guess.
Forecast was for fog and oh yeah it came.
We were parked out at the GA apron and as I walked out from the big TNT shed towards the plane I could sense something moving behind me. I turned around and a huge bank of fog slowly rolled over the top of the shed like a crashing wave. I'm not sure how long it took because I was mesmerised by how it all looked, but not too long afterwards it was barely possible to see the glow of lights on those big light poles in the middle of the apron.
Mate of mine just managed to land on 14 before the fog got that far, and because he couldn't see where to go he had to wait about twenty minutes for a ground car to get out there to guide him back. Took about 45 minutes from landing on 14 to park the plane at GA.