Probably North Carolina, as at the time, my final prediction map had John Kerry winning North Carolina, Ohio, Iowa and Florida.
Why was NC a surprise to you? There wasn't a single poll showing Kerry close to Bush in NC. The closest I believe was an eight pt margin following the selection of Edwards.
I thought that the selection of John Edwards as John Kerry's running-mate might have flipped North Carolina, albeit barely. Granted that at the time I was only 10 and knew little about politics and the voting patterns of specific states.