'Brian' for me but I loved the inventiveness of 'Grail' due to the limited budget (clonking coconuts shells as a horse substitute, painted cardboard cut-out of Camelot on the hill). What I liked about both films was that they were a series of sketches that formed a complete story set in an historical context.
This is why 'Meaning of Life' was a big disappointment to me as the Python team went back to an individual sketch format based around a concocted theme to try and stitch it all together. Saying that, it still contained some great skits.