For certain, 30mph on my hybrid on a steepish descent. I probably could've gone faster, but the front wheel started the death shimmy.
There are two other times I may have hit that speed or even faster, but one didn't have a computer and the other the pavement was rough enough I couldn't take my eyes off the road to look. I'm the opposite of a speed demon; I was trying to get more comfortable with it. Now I'm older and fatter, and my insurance is worse, so brakes for me.
There was a memorable (to me) descent where I rode the brakes hard the whole way down, keeping it to 15mph. It was the end of January so the road was wet with snow on the edges, tree debris was everywhere, and the pavement was chunky. I figured I'd rather crash at 15 than 30+ (I didn't crash; yay!).