Which reindeer on Santa Claus's sleigh is the fastest?

There are 8 reindeers that carry Santa's sleigh, 9 if you include Rudolf. All of them are the same size but if you listen to all the jingles and folk tales you'll hear that Rudolf was the extra reindeer, and the most special of them all. He was Santa's favourite and he led the reindeers to all destinations, so, in theory you could say Rudolf would be the answer to your question.