What is the steepest roll-off achievable for an analogue low-pass constructed with a single op-amp?
You can make a high-order low-pass filter with no op-amps at all. In principle you could obtain any roll-off rate you like, given enough pi or T sections. You could then add an op-amp output buffer to make your filter a "one op-amp" filter if for some reason you'd like to do that.

But high-order filters tend to be very sensitive to small changes in component values, so they often require hand-tuning each circuit to achieve good performance, and the performance could drift as temperature changes.

The benefit of using active filters is that the op-amp allows low output impedance and high input impedance, reducing the loading effect of one stage on the next. This means you can cascade several first or second order filters without much interaction from stage to stage, and reduce the need for tuning. If you try to make a high-order active filter in one stage, you're giving up a key advantage of using active filters.