Adrien Brody -- After winning for "The Pianist" in 2002, a year when all four other nominees were more memorable (well, maybe not Nicolas Cage), he played leading roles in overblown duds ("King Kong," "The Village") and has settled back into work as a character actor in movies hardly anybody sees ("Detachment," "Predators"). Brody, the ultimate one-shot wonder, will next play Flirty Harry in "InAPPropriate Comedy.".
Nicolas Cage -- Speaking of The Rolling Cheeseball, he now careens from starring parts in junk ("Ghost Rider: Spirit of Vengeance") to blah roles in franchises that can't be re-written to exclude him (the upcoming "National Treasure 3" and "Kick-Ass 2"). Nowadays, he rarely even shows the manic energy that earned him the nickname "Nicolas Rage," and he NEVER shows the depths that won him the 1995 Oscar for "Leaving Las Vegas."
Mel Gibson -- Speaking of 1995, that was the year Gibson won as director for "Braveheart." You can debate whether it was the best-directed film that year (I'd put "Babe" above it), but what has he directed since then? The violent, one-note "Passion of the Christ," the incomprehensible "Apocalypto" and...nothing. There may BE a good director somewhere inside Gibson's head, but we're probably never going to find out.
Christopher McQuarrie -- "The Usual Suspects" remains one of the cleverest thrillers I've seen and justly won best original screenplay for 1995. I looked forward intently to the things he would do next. And they were: "The Way of the Gun," some TV and the dreadful "The Tourist." Otherwise, he has mostly nestled in Tom Cruise's pocket, from "Valkyrie" and "Jack Reacher" to the proposed "Top Gun 2" and "Mission: Impossible 5." Zzzzzzz.
Renee Zellweger -- The supporting actress category offers many people who didn't live up to potential, from Mira Sorvino to Marcia Gay Harden. But I'm going with Big Z, who won the 2003 award for her Granny Clampett knock-off in "Cold Mountain." Except for animation ("Shark's Tale," "Monsters vs. Aliens") and the first "Bridget Jones' Diary," Zellweger and her movies have consistently redefined "mediocrity" over the last decade.