Look at the film techniques in most other films from the forties and you might get it. For its time, it tried a multitude of different stylistic approaches to cinematic storytelling, mainly in its photography and editing - Orson Welles might as well have invented the tracking shot himself. It's "the best" because, without it, contemporary cinema would not be the way it is.
I understand that for its time it's very advanced but it doesn't mean by my personal taste it is "the best film ever". Just because something was created/invented that led to improvements in the future it doesn't automatically give it the title of "best ever". If Orson Welles hadn't have done the things he did no doubt others would have eventually. I'm not arguing that it's technically a good film but I would disagree with best film ever because that is entirely by taste. Now if they were to change it to being something like, "most pioneering film ever" or "the best technically made film ever" I may be inclined to agree.
who died and made you the mubi law maker? i posted this on my filmography but as you should be aware it then also posts it to the film page which cannot be helped. i think there needs to be some separation of those features. some people might want to follow what peoples thoughts are as they watch a film and i wanted to comment before i forgot about it entirely. my comments still stood the same after the film as any one can read from the subsequent comment above yours. any ways, ho-hum.