I don't think I ever believed in Santa Claus. It was a fascinating subject for me as a kid though
Considering the world had a population of over 4 billion, of which only about 30% celebrates christmas, which was only a ~1995 year old holiday - if that - I always thought the concept of someone so biased and fast was pretty stupid. If it takes him about 10 minutes per house, he couldn't possibly even do a whole city, never mind the whole world, in one night. Geography class showed that there's no usefully habitable location as the North Pole. So the only way Santa could exist would be if he lived in the South Pole with Penguins. I also wondered how the postal service could deliver to the North Pole without a postal code.
I always believed in the Easter Bunny though.