Zapiecek is a Polish restaurant chain that has become the standard reference for pierogi in Warsaw. The Old Town branch on the market square is the most central. The menu runs to dozens of pierogi varieties — the traditional ruskie (potato and cheese) and kapusta z grzybami (sauerkraut and mushroom) are the benchmarks, but seasonal and creative varieties change through the year. Portions are generous and prices are fair for the location.
Other branches are less touristy and slightly cheaper; the Nowy Świat location is a good alternative if you're walking the Royal Route. Sit-down service and takeaway both available.