My lowest position is 49,965th (out of 58,365) which was a partial segment of Sawyer's Hill in Richmond Park - in fact all my worst ten segments by placing are in that bit of Richmond Park. Did those during RideLondon last year and in my defence it was very crowded at that time with people already walking up the hills and others pulled over to the side with punctures or waiting for friends, which narrowed the already narrow road further. No matter how fast you wanted to go, there was no ability to overtake, so you just sat behind whoever was in front of you and everyone did the same crawling pace for that mile.
However that's not my worst segment - it only ranks so low because of the sheer number of people who've done it. Thanks to Veloviewer, I can see what my poorest performing segment in terms of percentile is and it is the "Descent of White Hill" in Chesham, Buckinghamshire. A 10% descent (reaching about -15% in parts) that has a 90 degree turn at it's steepest bit and ends abruptly at a roundabout in the heart of a busy town centre. I went down it gingerly with full brakes on for most of it and so ended up as 2,030th out of 2,083, almost a whole minute behind the KOM (who did it in 30 seconds)!