Depends on what your definition of a good show is.
Obviously a lot of stock goes into the lead singer, as he is the main voice you hear. If going by that, I have to say 2008 was worst. Songs like Leave Out All The Rest were cringe-worthy at times. His screams sounded very thin also.
2009 his singing started to improve a bit, especially during the DBS tour (except for that first performance of Into You, good lord. XD ) Yeah the first performance of New Divide was bad but it got better as time went on.
2010, Chester's singing sounds pretty on point. I'm pleasantly surprised. It seems like he's accepted that his voice isn't what it used to be and isn't trying to push it so hard anymore (bridge of Given Up, lowered key of The Messenger, etc.)