I have huge respect for the likes of Shankly, Ferguson and Clough who either picked sleeping giants up of their knees and took them back to the top or, perhaps more impressively, took mediocre clubs to unprecedented heights.
As for the best European manager it's so hard to choose. Stein took 11 local boys to a European Cup, that is something that will never be repeated. Clough won Nottingham Forest back to back European Cups, Trappatoni is only behind Fergie in terms of success, Shankly made Liverpool what they are/were, Fergie rebuilt Man Utd and turned them into arguably the biggest club in the world, Mourhino has an outstanding record, while there are those who will laugh good old Sven has a hell of a record and i doubt there will be many who have a more successfful start in mamagement than Pep Guardiola.
Very hard to choose but for sheer longevity i'd go for Sir Alex.