Vastly Diminishing Returns
Imagine a number line. Now, imagine that the zero point on the line represents a completely average film, and that the farther to the right you go the better the film (so, say, The Godfather, would be way off to the right), and that the farther to the left you go the worse the film (so, say, Leonard Part 6, would be way off to the left.
Now, what series of films, what franchise, would you say has traveled the farthest along that continuum? What series has devolved (or, if there is one, evolved) the most throughout its history? I'm not sure what the answer is, but my answer is the Superman franchise. It's hard to believe the sheer distance in quality between the first and last of that series. It's even harder when you realize that it's not really as if there was some kind of major tonal shift that could account for it. Sure, in the last two installments the comedy became too prevalent, but that comedy was very much in evidence from the first film (witness Gene Hackman's less-than imposing Lex Luthor)--it wasn't a new development. And yet the first film is so very far ahead of the fourth and last. Why? What happened to cause this severe jump? It's a mystery (to me).
I'd be pleased as the proverbial punch to hear any others' thoughts on potential nominees for biggest quality jump ever. And I'd be very interested to hear if anyone can think of a series that has increased very dramatically in quality as it's progressed. The Harry Potter films come to mind, but, to me, while the third is clearly the best so far, the first has much to recommend it. They have gotten better, but the jump hasn't been remarkable.