Optocoupler - Emitter follower for driving an optocoupler
This depends largely on what size capacitor you're discharging and how long you have to discharge it, as well as the residual voltage you can tolerate.  If it's a 100pF cap you're discharging, I'd say you can do it with the schematic above; there will be a high peak current, but not long enough to significantly heat the driver.  If it's a 100uF cap, you'll probably want to include additional drive or add a series resistor (which will slow the discharge rate).  "The most rapidly" isn't a workable spec unless you're doing high energy EMP research, which is far more expensive than this; you need a number to work to.