I went to Zapiecek on Świętojańska on the recommendations of my hotel’s staff when I asked where to get good pierogi. It is very small, I don’t know if they take reservations, but if you’re a group, it might be better to try to book. The staff is very friendly.
I loved it so much I went twice during my short stay in Warsaw, I tried the simple meat ones (they can be blend with no topping, so choose wisely), the Russians (potatoe and cottage cheese) with bacon and the ones with camembert and blue cheese. Those were awesome! So awesome I took a big portion of those on the 2nd dinner, I tried with the strawberry sauce they recommand, but I definetely am not into sweet and savoury (but it comes on the side, so it’s fine). I also tried some sweet ones, with cottage cheese in vanilla sauce, but it wasn’t the best combination.
To sum up, this place is great for pierogi!