The flat itself is really nice. Best is probably the location and the space. Second bathroom was a plus when staying with many people.
A few things can be improved though: the mattresses were very dirty and one bathroom smelly all the time (improve this and we'll give five stars next time). Telling that there were enough dishwasher tabs would have been great as well. More kitchen hardware will be nice when staying with more people. Having a washing machine but no washing powder was slightly suboptimal.