Based on my experience of travelling and living around Europe, I would like to make a list the the best cities by category, rating them respectively 1A, 1B, 2A, 2B, 3A and 3B. Note that I have listed Nordic countries or Ireland, and have only been to Praha and Budapest in Eastern Europe. Basically, the numeral ranking is about the overall beauty/quality, while the letters refer to the length of stay to fully enjoy the city (A the longest, C the shortest).
1A : outstanding cities with enough sights for 1 to 2 weeks
London
Rome
Florence
Paris
1B : same as 1A, but with sights for less than 1 week (usually 2 or 3 days)
Salamaca
Seville
Venice
Nice
Oxford
Cambridge
Praha
Vienna
Istanbul
1C : same as 1B, but with sights for just one day
Cordoba
Caceres
Evora
Pontevedra
Santiago de Compostela
Carcasonne
Monaco
Pisa
Lucca
San Gimignano
Siena
Lille
Luxembourg
Bruges
Maastricht
Bath
York
2A : very good destinations with sightseeing for several days, but less impressive overall than those ranked 1A.
Brussels
Amsterdam
Copenhagen
Stockholm
Barcelona
Madrid
Lisboa
Budapest
Athens
2B : Same as 2A, but with sights that can be covered in 1 day
Granada
Merida
Milano
Marseille
Nimes
Avignon
Montpellier
Antwerp
Namur
Delft
Leiden
Brighton
Stratford-upon-Avon
3A : Less beautiful cities with enough exceptional attractions for several days
Berlin
3B : same as 3A but with sights that can be covered in 1 day
Bristol
Portsmouth
Salisbury
Canterbury
Munich
Frankfurt
Regensburg
Bonn
Trier
Aachen
Ghent
Gouda
Den Haag
1A : outstanding cities with enough sights for 1 to 2 weeks
London
Rome
Florence
Paris
1B : same as 1A, but with sights for less than 1 week (usually 2 or 3 days)
Salamaca
Seville
Venice
Nice
Oxford
Cambridge
Praha
Vienna
Istanbul
1C : same as 1B, but with sights for just one day
Cordoba
Caceres
Evora
Pontevedra
Santiago de Compostela
Carcasonne
Monaco
Pisa
Lucca
San Gimignano
Siena
Lille
Luxembourg
Bruges
Maastricht
Bath
York
2A : very good destinations with sightseeing for several days, but less impressive overall than those ranked 1A.
Brussels
Amsterdam
Copenhagen
Stockholm
Barcelona
Madrid
Lisboa
Budapest
Athens
2B : Same as 2A, but with sights that can be covered in 1 day
Granada
Merida
Milano
Marseille
Nimes
Avignon
Montpellier
Antwerp
Namur
Delft
Leiden
Brighton
Stratford-upon-Avon
3A : Less beautiful cities with enough exceptional attractions for several days
Berlin
3B : same as 3A but with sights that can be covered in 1 day
Bristol
Portsmouth
Salisbury
Canterbury
Munich
Frankfurt
Regensburg
Bonn
Trier
Aachen
Ghent
Gouda
Den Haag
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