20mph and 28mph work out pretty good. I have a 45kph bike- faster than that would be rather dangerous, but it's fast enough. I sometimes think a cheaper 20mph (32kph) bike with less power and weight would be pleasant; on the 45kph bike, you can't enjoy the scenery or hear the birds, it's too fast. 20mph would be as fast as I ever ride unassisted.
I don't even want to think of in terms of "[insert speed limit] bike", to me it makes more sense that your bike is however fast you ride it, and if it's an electric assist bike, you ride however fast your muscle plus the limited power of the engine ride it. Then it's only a matter of finding the right balance of power that makes the thing useful without turning it into an insane crotch rocket.
