I was having this discussion with my friends one drunk evening in Prague. I argued Celtic because of worldwide fan base, history and success. Although I do see that Liverpool are a big club, I personally see Celtic as bigger, with Man Utd the second biggest in Britaina and Liverpool as the third.
Going on a world wide fan base (glory hunters) surely ManureUtd would top the heap.
Celtic still attract a number of fans for reasons other than football (as do Rangers and linfailed), but I don't really see the point of this discussion, unless we resort to getting out a ruler.