On my current bike (MTB), 38.4mph on a road down a huge hill. Unfortunately the rush didn't last long as the big downhill fed into an equally big uphill, and the momentum took me about 2/3 of the way up before I had to work.

As a teenager I clocked 45mph on a hill near my parents' house, the kind of hill I'd ride up (even if that meant push up) just so I could get into a high gear and pedal like fury on the way down.

On another hill as a teenager I was riding a Grifter (the first bike I had that had gears) and a speedo based on a wheel resting against my front tyre. That clocked me at 35mph when I went off the road onto gravel. I'm still not quite sure how I stayed upright to get back onto the road.