united winning last year doesn't mean that united is far and away the best team in europe. in fact, there isn't much that separated chelsea and united last year (i.e. very, very little). i will consider united the best team in europe if they win, but that doesn't mean that i consider them much better than barcelona
a one off game doesn't really mean anything. winning consistently across all competitions means a whole hell of a lot. there may not be anything that really separates barcelona and united, and that's not really why i've argued in this thread