What is currently the highest slew rate of any available op-amp?
It's kind of an odd question. According to Digikey the fastest opamp slew rate is currently 24V/ns. ADL5569

But keep in mind the context - opamps are intended to have well-controlled, low-noise, linear outputs. This means that their slew rates are always "held back" to allow for good stability.

If you use digital circuits, like fast logic gates, the maximum slew rate is far higher. For example, I have worked with digital logic that has a slew rate of about 100V/ns.

Ultimately, the slew rate will be limited by the RLC characteristics of your circuit. And that is totally dependent on what you are trying to use the circuit for.