Worst DLC thus far for me has got to be the Pinnacle Station DLC for Mass Effect. Gee, just what I wanted, a DLC that is really only beatable if your a soldier-class. Anything else is like trying to climb stairs with your chin.
However, I am going to give a special shout-out to ANY DLC which is in essence a key to open content ALREADY on the damn disc. I'll grin and bear it when they say they couldn't fit it in because of scheduling issues, blah blah blah... whatever. But when I'm paying extra money for content I already have in my possession, esp. when I already went above and beyond and for example bought the collector's edition (ironically happened to me twice now with RE5 and Bioshock 2), then yeah, I'm only compelled to give said game developers and producers a hearty two finger salute...
...and probably do the exact stupid thing again when they release Bioshock 3 and RE6...