I agree that Life of Brian is the best of their movies. I can't for the life of me remember much of The Flying Circus, so I'm not in a position to say it's the best of their work.
I thought that The Holy Grail and The Meaning Of Life were slightly too all over the place. The Life Of Brian had a good solid plot, great satire and generally just the best jokes.
I will be getting this when I have the money to though, naturally.