Check out BooMan's year-by-year descriptions and come back to enter your vote.
Hands down, 2003 and 2005 were the worst years of the Bush administration. And if I had to choose one of those, it'd be 2003. The darkest year of the dark ride. Easily.
UPDATE: I went with 2003 over 2005 because I was thinking in terms of Bush popularity, specific actions by the administration and, looking back, 2003 seems so early -- in the heart of the darkness, really.
We had the invasion and occupation of Iraq, the president's soaring popularity augmented by his U.S.S. Lincoln flightsuit thing, followed by a violent insurgency fueled by the president's "Bring 'em on!" statement.
Torture at Abu Ghraib and Guantanamo was underway even though we wouldn't find out until 2004. Same with the illegal eavesdropping program. Dissent -- especially against the war -- was still being silenced and marginalized.
Progressive talk radio didn't really exist in its present form and the liberal blogosphere was a fraction of its present size. The press was mostly still asleep. At least Katrina, in 2005, woke them up a little.
Speaking of Katrina... While it was an unprecedented disaster, it actually had the positive benefit of exposing the administration as the incompetent bastards the rest of us had recognized from the beginning. So while it was a terrible event, there was at least something of a positive consequence in that it may have helped to rescue the world from the Bush Republicans further expanding their power.