You need to specify more information. This depends on
- signal consumer requirements
- what bandwidth is needed?
- is error at a particular sampling point important?
- is this being used by a digital or analog circuit?
- is linearity important?
- signal producer capabilities / signal properties
- what is power supply voltage?
- what is source slew rate?
- what is source waveform timing?
If I just want to look on a scope, a 1us slew rate for a 10us pulse might be fine. If I'm sampling at particular instants in time, maybe the error during a particular sample might be important.
If your consumer is a digital circuit, you should be using a digital buffer, not an op-amp.
Also I challenge your problem statement:
Given pulse signals generated by a uC, what kind of slew rate on an op-amp will you need to reproduce the waveform cleanly?
Microcontroller outputs do not have "clean" voltage levels, so it really doesn't make sense to try to buffer their output waveform "cleanly".
If you need to maintain precise timing and particular reference voltage levels rather than the somewhat-indeterminate "Vol" and "Voh" output voltage levels of a microcontroller, which can drift, bounce, etc. as long as they stay within spec, then I would highly recommend using a fast analog SPDT multiplexer. (FSA4157 / NLAS4157 are really good choices) Hook the A/B inputs to stiff reference voltages of your choice, and hook the switching input to your microcontroller output. The multiplexer output is your analog output signal.
I can't think of any good reason whatsoever to spend money on a fast op-amp to buffer a cruddy microcontroller output signal.