Here's a tough category to pick. The nominees are Mad Damon (Invictus), Woody Harrelson (The Messenger), Christopher Plummer (The Last Station), Stanley Tucci (The Lovely Bones), and Christoph Waltz (Inglourious Basterds).
Over the years, there have been several different scenarios that lead to a win in this category, and we have all of them represented here. Sometimes Supporting Actor is used as a defacto "lifetime achievement" award for well-regarded seniors and Christopher Plummer fits the bill. Sometimes it goes to a leading man, like Mat Damon, who takes a lower-profile role (not unlike a racehorse dropping in class and running away with the race). Many times, the Academy rallies support for an actors like Stanley Tucci, who has been doing solid work for years but haven't had the opportunity for a nomination. Comebacks are always popular and Woody Harrelson had an outstanding year after a fallow period. Finally, voters like to jump at the chance to recognize a relative newcomer, like Christoph Waltz.
With all that in mind, I'm going to pick Christoph Waltz. Virtually unknown on our shores until he was "discovered" by Quentin Tarantino, Waltz has had a successful career in his native Austria for years. It's a classic case of the actor fitting the role perfectly. Waltz has already taken the SAG, the Golden Globe, and several other top prizes, and it's a role where he has to speak four different languages fluently. Also, he plays an over-the-top bad guy and, while that would be a detriment in the Lead Actor category, it's a plus in the Supporting Actor race. His next biggest competitor is Stanley Tucci, but I'm going to pick Waltz for the win.