It's easy to pick on low budget stuff, so my nomination is the Rancor from RETURN OF THE JEDI. A budget of ninety squillion dollars and you can still see the matte lines. Then, twenty years later, Lucas does the Special Editions and you can STILL see the matte lines. Ten years after that the DVD retinkerings come out and, yup, can can STLL see the bloody matte lines! Insanity.

EDIT: The same goes for the garbage mattes around the TIE fighters in the shoot out with the Falcon scene in A NEW HOPE.  There's no excuse.