Bugatti Veyron or the McLaren F1...i think its the Bugatti though
I believe it falls to the Bugatti Veyron 16.4 Grand Sport. Price is supposed to start at $1.9 million when available in 2009. I think the McLaren F1 was the most expensive when it was in production but I could be wrong.
i thought the mercedes clk/lm was the most expensive production car according to guiness world records.