I recommend Ondrej and Monika's place without reservations.
The apartment is very clean, well equipped, comfortable, bright and has a familiar atmosphere - you feel at home. It is located in a quite and safe residential area of Prague, close to public transport (tram stop 'Marjanka') and to the Castle (four stops with the tram).
The room is very clean and also comes with a table with chairs, which is good if you have some work to do.
The bathroom is very functional (there is even a window!) and the shower has a strong water jet.
Monikas was very communicative and responsive to my enquiries both before and during my stay. The internet connection also worked very well and, importantly if you like cooking, there is a fully furnished kitchen that can be used.
Overall, I am very satisfied with my choice and I would most likely choose Monika's apartment if I happen to be in Prague again - it is also a good value for money.