The A34 from Bicester to Oxford... at 5pm on a weekday.
The traffic was still flowing at full speed, but dense as it could be.
A close second was ending up on the A40 Westway at night time on a commuter bike. Just past Royal Oak, and the hard shoulder disappeared. (Until 2000, it was the A40(M)).
It has to be said, I've done Oxford-Abingdon on the A34 at 7am and it was actually pleasant. Not one in-lane overtake! Time of day is often a huge factor.
I'm tempted to do a dash up the M40 at 4 am, a hard shoulder would be a luxury.