I've only seen the two big ones, and I'm surprised by how difficult this question seems to be for people. Life of Brian and Holy Grail are very different, most people I've discussed this with IRL tend to have a strong preference between the two. For me, it's clearly Life of Brian: it's possible Holy Grail has more laughs, and it certainly has more quotable lines, but Life of Brian hangs together much better as a film, whereas Holy Grail is really just a series of great sketches. Also, "Always Look on the Bright Side of Life" is one of the best endings to any film ever.