AAA Taxis, which are easy to recognise as they are usually yellow cars, have a good reputation for not ripping people off, if a club or hotel gets a taxi for you it will more than likely be an AAA taxi as they are known as a reliable company. You will normally be able to find their phone number on a card in hotel receptions or in promotional literature that is normally in your room.
Easy Transfers not only do airport transfers but also transfers around Prague and most other places in Czech Republic, a well known company with a good reputation so good to use. They have fixed prices so there is no suprises when you get to your destination.