Generally, it is not advisable to take a bus tour in Prague, because the sightseeing hub of the city, mainly Charles Bridge, the Old Town and Hradcany, can not be accessed by bus. Actually, many bus tours in Prague are simply walking tours which utilize buses to pick people up and transfer them. Thus, if you do insist on touring by bus, you will either take a hop-on/hop-off bus tour, that is not reviewed well on the forum on Trip Advisor, or you will take a combined bus and walking tour.
Bus tours are better suited to day trips away from the city and, in this instance, are good if you wish to make your trip free of stress, rather than catching a public bus or train.
1) HopOn HopOff – webiste
2) CitySightseeing Bus Tour website