It varies with the power or output voltage. The maximum slew rate at, say 200 Watts into 8 ohms at a maximum frequency of 20 kHZ is 7.1 Volts per microsecond. So anything that's closest to this number is the fastest for a 200 watter. If you're interested in the math:
Vpeak = sqrt(200*8)*1.41 = 56.4 Volts
SR = 6.28(20,000)*56.4/1000000 = 7.1
Vpeak = sqrt(200*8)*1.41 = 56.4 Volts
SR = 6.28(20,000)*56.4/1000000 = 7.1