London
Florence
Paris
Venice
Rome
Athens
Vienna
Salzburg
Barcelona

Paris
Rome
London
Venice
Salzburg

Never been to Vienna, Florence, Athens or Barcelona and this is very hard!!! These cities are so very different! For instance, I adored Salzburg but it is not a major city like the others. I don't think Venice is either. I liked London, but I didn't find it as culturally different as Paris or Rome. However, I was not there long. I think I need to go back and spend a week in each city and then I MIGHT be able to post something more knowledgeable. Of this list, I've been to Paris more than the other cities so it really is biased toward it. Yep, I definitely think further in depth research is required to answer this question.

I've been to ALL the cities, most more than once. Rome wins hands down! I like it for the history, food, culture. Saltzburg is also beautiful, London is fun and lively.

Rome
Athens
London
Salzburg
Florence
Venice
Barcelona
Paris
Vienna

Vienna didn't impress me, but I only visited once, so sometimes it takes several visits to appreciate.

That's tough because everyone has different interests, but here goes;

Paris
Rome
Venice
Florence
London
Athens
Barcelona

I haven't been to Vienna or Salzburg.

Florence
Rome
Salzburg
Paris
Vienna
London
Athens
Venice
Barcelona

My favorites are:

Paris
Venice
Florence
London
Rome
Barcelona
Vienna
Salzburg

I have never been to Athens. And Seville, which is not listed comes right after London.

The OP didn't really indicate what he/she planned to do about Barcelona so, did we help or not?

I will rank them seperately.

Big cities:
1) Rome
2) Paris
3) London
4) Vienna
5) Athens- never been there

Tourist destinations
1) Venice
2) Florence
3) Barcelona
4) Salzburg

If you want rank Barcelona as a big city I would put it just behind Vienna. Though I love Madrid and would put it number three.

This is nearly an impossibility! I don't know YOU and you don't know ME! Each city has it's own charm and appeal. Salzburg is tiny compared to the other major large cities that you have listed and cannot be compared with those. Florence and Venice are also in a different class and are not as metropolitan, compared to the others that you have listed.