Internet cartoonist, Dan Meth, created this chart of movie trilogy relative goodness.
In my opinion, his ratings are stunningly accurate. Yes, X-Men III and Terminator III were utter crap. However, I liked Batman Forever (the third installment) better than he did, especially given that it was followed up by the single worst movie of all time, Batman and Robin:
Also, Superman III tends to be undervalued by critics because of its light-heartedness. Not only does it feature comedy Demi-God Richard Pryor, but included this scene that scared the everloving crap out of me as a kid:
Anyway, well done Meth Man.