Manchester United, Manchester City, Tottenham Hotspur, Arsenal, Liverpool: Europe's Biggest Derbies By Combined Average Attendance
These are the combined average attendance figures for the whole season 2011-12 (i.e. not just for the derby games themselves - in some cases the clubs are in difference leagues). This gives a better indication of a club's loyal support throughout the season, not just for the big games. Of course it would be wrong to suggest that the bigger the attendance, the fiercer the rivalry between the two teams. The list merely provides a rough guide and there are some problems with it, which I've done my best to outline below.
For the sake of simplicity I've only included teams whose grounds are within 5 miles of each other. This way the list looks at clubs within the same 'catchment' area. We should be clear that the Revierderby (Borussia Dortmund and Schalke) is by far the biggest in Europe, with a combined average attendance of 141,739 - however their grounds are 17 miles apart, making it less of a local affair than, for example, Liverpool vs. Everton. Similarly Hertha Berlin and Union Berlin don't make the list even though they have a combined average of 69,573 as the two stadiums are 14.4 miles apart. Newcastle United and Sunderland are 10 miles apart with a combined average of 89,030.
All figures are from www.european-football-statistics.co.uk - unfortunately the site doesn't have up-to-date figures for the Turkish clubs.
London (and to a lesser extent the West Midlands) poses all sorts of problems in terms of the amount of well-supported teams and the character of the rivalries. The grounds of Chelsea and Fulham are practically nextdoor to each other, but there isn't a big rivalry between the two clubs. The Manchester derby may top the list but London is arguably the biggest football city in Europe.
Many of the derbies are lopsided. Generally it would seem that the smaller the gap between the two average attendance figures, the bigger the rivalry between the two clubs. Milan, Glasgow, Seville and Rome arguably host the fiercest one-city rivalries in the list. The bigger clubs (in relation to their local rivals) tend to look elsewhere for their main rivalries, with Manchester United vs. Liverpool and Barcelona vs. Real Madrid the classic examples.
Another option would be to rank the derbies by the attendances of the lesser supported clubs, in which case the top five would be (1) Manchester, (2) Glasgow, (3) Milan, (4) Madrid, (5) North London. This way the lopsided Barcelona derby falls out of the top 5 and the Glasgow and Milan derbies move further up the list but the Manchester derby remains the biggest.