I actually enjoy driving on the M25. (Well, most of the time, anyway!). It's joining it or leaving it which can be really tricky though. For example, if you're travelling anti-clockwise and wanting to join the A12 northbound (at Jn 28) the lane markings on the slip road don't seem to match up with those on the roundabout (or with any idea of common sense). Motorists all have different ideas of the correct way to go round the roundabout, which seems to be a recipe for disaster. Then, as you exit, you find that your path merges with traffic from your right (with no warning) and then, immediately afterwards, with traffic from your left.
That's not the only junction which scares me. About half of them do!