Imre Kovács
Google
Nice place at a somewhat hidden location (you have to go under the street level, but worth it). Their specialty is the donuts, as the name tells and those are badass, indeed. They have some strange but awesome combination, even with bacon.
The filled ones are light, but still filled with lots of stuff, for around 500 HUF. They have really good coffee and tea as well and the staff is nice and friendly.