Speed cars is my favorite taxi company in Prague. The prices are very low, the drivers are always on time and the cars are almost always clean. You can make a reservation by phone or sms. You receive a confirmation by sms letting you know the exact time they will arrive and the car that will be there. The only negative is the drivers can be fussy about using the credit card machine, claiming they don’t have a terminal when in reality they just don’t want to use it.