Andrew Gelman quotes from the best possible rejection letter from a journal (sent to Charles Babbage):
It is no inconsiderable degree of reluctance that I decline the offer of any Paper from you. I think, however, you will upon reconsideration of the subject be of the opinion that I have no other alternative. The subjects you propose for a series of Mathematical and Metaphysical Essays are so profound, that there is perhaps not a single subscriber to our Journal who could follow them.
I encourage all journals to adopt this as the standard form letter for rejection.