The following is a ranking of the 11 most successful European clubs in history. The criterion is quite simple and couldn't be any more objective: number of (official) trophies won.
To make it a little more selective, clubs who haven't won the European Cup/UEFA Champions League in the past 25 years were excluded, since, as far as I'm concerned, a club who hasn't been successful in the most important European competition for that long doesn't deserve to be considered among the European elite.
This led to the exclusion of such clubs as Celtic (91 trophies), Glasgow Rangers (113), Sporting Lisbon (41), Benfica (64), Steaua Bucharest (50), Levski Sofia (55) and CSKA Sofia (41), who would have otherwise made the list.
Still, I believe it was a fair criterion, which made this ranking more selective and thus adequate.