Furthest north I've been is somewhere on the A93 between Banchory and Ballater. This was in August 1995, shortly before the Worldcon. Of course if I'd been driving in the opposite direction I'd have been on the other side of the road and therefore a couple of yards further north.
Furthest west I've been (on the ground) is probably on some taxiway somewhere at Washington Dulles International Airport, on various occasions between 2003 and 2006.
Furthest east I've been is where the where the Great Wall of China meets the sea sometime in May 2002.
Furthest south I've been is probably Huntington VA metro station, just outside Washington DC. I've been nearly as far south as that in China, but most of the tourism was either in Beijing, just a bit too far north, or along the wall, further north still. Furthest south here in Europe would be Bilbao, laughably nearby.
Later this year I hope to be going to New Zealand and Australia, which will smash my existing records.
[Edit: Northernmost is 644km away; Westernmost is 5890km away; Easternmost is 8350km away; Southernmost is quite near to Westernmost, at 5875km.]