Ngam ngam enough for 4x 92k KF.
As mentioned by bro knwong, every city is unique and has different thing to see / do.
Also europe is very small, you can go around multiple cities / countries easily by train, don't restrict yourself to one.
London / Paris are best bet.
Milan / Rome if you like Italy.
Barcelona if you like Spain / Portugal.
Frankfurt / Munich are great transit hub to other European cities.
Zurich for Alps.
Missed out Amsterdam, which is another great city.
If you can't make up your mind, let seat availability determine your destination.
I am just thinking of the entry city that uses the least KF miles. So if London is the cheapest to enter and exit thru London and I will travel between the cities by train.