iCheckMovies.com has organized my film canon addiction into manageable form. It has a broad number of canons to choose from and follow, with nice shiny awards to win at various levels of completion. Of course, the number of canons causes debate over which ones are better than the others.
We're really dealing with 3 different concepts here.
1) The individual critic's list
2) The critical consensus
3) The measurement of the critical consensus
1) The lists made by individual critics vary widely. Roger Ebert has a celebrated one, that stays mostly in the mainstream. Jonathan Rosenbaum's list of 'Essential Films' is one of the best known examples of unorthodoxy. He includes a large number of obscure films and even some pretty bad ones, but excludes Casablanca.
Both Ebert's and Rosenbaum's lists are good ones. They each champion overlooked films and criticize via omission ones they feel are overrated. Each can justify their selections coherently and intelligently.
2) 'The critical consensus' functions similarly to a scientific consensus. It is not a standard of absolute truth, but a good way to measure knowledge in the field.
3) The success of #3 is different than that of #1. While it's perfectly OK for Rosenbaum to neglect Casablanca in his own list, a 1,000-film-long #3 list that did not include it would be in error.
Citizen Kane's ranking is a good shibboleth to test the legitimacy of a canon. Not every critic thinks Citizen Kane is the greatest movie of all-time, and I'd be surprised if a majority ranked it #1. But the general critical consensus is that Kane is among the greatest ever made. If it appears too many slots down, the discerning reader immediately knows he's not dealing with professionals (or educated amateurs).
There's a distinction between arguing with #2 and arguing with #3.
For example, I see the list at The One-Line Review and think that The Seven Samurai is grossly overrated. But, I know that the general consensus is that The Seven Samurai is one of the greatest movies ever made. So I regard The One-Line Review's canon as a good list. My quarrel is with #2.
I see the list at IMDb and think that Se7en is grossly overrated. I know that the general consensus is that Se7en was a good movie but certainly not one of the 30 greatest movies ever made, or even the 500 greatest. So in this case, my quarrel is with #3.
This doesn't mean that individuals who think Se7en is one of the greatest films of all-time are necessarily illegitimate critics or that their personal list is illegitimate - it simply means that the IMDb list does not reflect the current consensus of people who know film.
A #3 canon is most useful for novices who are beginning a film education, who want to measure their own opinions on film or who are just looking for something good to watch. A #1 canon is most useful for those who have a critic they like, are looking for films 'beyond the canon' or are looking to engage in intelligent exchange in criticism.