Well fwiw Grail is rated higher on IMDB than LoB. And I can agree that some (a lot?) of the skits in Meaning of Life are pure gold, but overall it lacks the narrative that ties both Brian and Grail together.
Life of Brian being rated as one of the best British comedies I can understand if only because it had more of a broader appeal (at least more widespread coverage thanks to the religious hullaballoo).
Grail on the other hand, may appeal to the cult market more and be more popular on that front (being the type of people who might vote on IMDB).
I'm grasping at straws here, but at least in my books the religious aspect of Brian (combined with the overall quality of the film) would lead me to say that it's more important than the other two Python films.
It's also telling that the two films you regard as better than Life of Brian are both by the Pythons :)
