You're right on all fronts except for one, it's not the best film. it's the most important film. two different things. as a form of entertainment, entertainment value is a huge factor in each persons best. so for me, if you are going to call a movie the best it has to be universally loved by the majority of people who watch it. Citizen Kane is split down the middle, and that's only because most people will never bother to watch it. it's not a film for everyone so much as an acquired taste. While no other film will ever progress the way films are made as much as Citizen Kane, many films have far engaging stories.
For example, the modern day equivalent to citizen kane, (and this is going to spark a lot of hate, im sure)imo is Avatar. It completely changed the way we watch movies and advanced special effects, specifically 3d effects leaps and bounds beyond anything we had seen before. the story was decent enough, but nothing special. it had a social comentary, but many view it as a shallow action film, and most people who watch it claim it as either a modern masterpeice or something impossible to sit through. (much like citizen kane) now, it may be the opposite side of that coin, but it's an example of how a film may not be oscar worthy simply because it advanced film making techniques. (most people dont bother are getting sick of 3d already) True, Kane did change movie making for the better, but if you look passed how it was made andpay attention solely to the story being told, there are better ones out there.
So, because the "best movie" is so based on opinion, to find a movie that can be universally considered the best movie, it has to be largely based on quality, make no mistake, but another big factor is popularity, whether people want to admit it or not. And so, to me, for the best of both worlds we have to fast forward to the 70s. Films like Taxi driver, Star Wars, The Godfather, and Rocky. thos are films still loved by young and old, and films like that are still important to cinema, maybe not as much so as Kane, but there's no denying the impact they had on movies. of those, (and im not saying those are the only films that are in the running, just what i thought of off the top of my head) I think The Godfather is arguably the best. I would certainly rate it higher than Kane in a review. However, that doesn't take away Kanes importance and influence. It just means it told a better story, and had better pacing, or was more enjoyable for whatever reason. I know I'm basically arguing semantics, but I hope you can see my point of view on this.