I'm not European but I've been living in Belgium for 1.5 years. Considering the cities that I've visited, I'd say:

1)Paris, 2) Milan, 3) Brussels, 4) Berlin, 5) Budapest. However, I feel that prices in Milan, Brussels and Berlin were almost the same, especially housing. So, for instance, a nice 40m2 studio in the center, furnished and expenses included, can be found for €800 (Berlin), €850 (Brussels), and €900 (Milan). On the other hand, I thought that Paris was going to be extremely expensive, but besides housing (that is indeed super expensive), things at the supermarkets and random restaurants were actually ok, almost the same as in Brussels, where I currently live.

What about Copenhaguen vs. Oslo? Geneva vs. London? Amsterdam vs. Paris? Is the most expensive European housing for "middle class" in London? Or maybe in Geneva? What about public transport? And some other interesting facts? Would be nice to create sort of a Reddit ranking for the most expensive and most affordable European cities to take into account when we travel.

Thanks!

Update:

Tier 1: 1) Montecarlo & Monaco

Tier 2: 2) Geneva & Zurich, 3) Oslo, 4) Reykjavik

Tier 3: 5) Luxembourg City, 6) Copenhagen, 7) London, 8) Dublin

Tier 4: 9)Amsterdam, 10) Stockholm, 11) Paris,

Tier 5: 12) Munich, 13) Hamburg, 14) Milan, 15) Brussels, 16) Viena, 17) Berlin, 18) Athens, 19)
Rome, 20) Madrid,

Tier 6: 21) Prague, 22) Naples, 23) Budapest, 24) Bucarest, 25) Lisbon, 26) Sofia, 27) Tirana.