I know lots of people are going to say the Lotus 49 or the McLaren MP4/4.

But for me it is the Ferrari F2002. A car in which Schumacher finished 1st or 2nd in every single GP that year (Malaysia, the only 3rd place was in the F2001).

9 1-2 finishes out of 14 races with that car as well. For a car to be that dominant, even with an average driver like Rubens Barrichello behind the wheel, makes it the greatest F1 car ever raced.