A Prague must! This IS a Czech pub. Go here. Loud, drinkers, dark wooden benches and pilsner fresh from the tank.
If you want to drink pure gold, the Tank Budvar here would be it. This pub has been around for a long time (1466).
U Houbaře (the mushroom picker) is not in the center, but well worth an easy tram ride.
Upstairs, downstairs, or the beer garden in ruins of a Gothic church, you can’t go wrong.