If you had to ask me...
It would be the Fantom Fury! I owned one when I was a little kid. That thing was amazing! It was sleek, quiet and pulled out LOTS of dirt from our carpets! Its dual-cyclonic technology (licensed to Fantom from Dyson) used 2 powerful cyclones to capture dirt and debris.
Sadly, my parents got rid of the Fury when we moved out of our apartment in the summer of 2001. In addition, Fantom made some disastrous business decisions and ended up going out of business later that year. Nowadays, these vacuums are hard to find. Even finding a Fantom in excellent condition is rare!
And that's why I love the Fantom Fury. It holds a special place in my early childhood, and I truly feel that it is a masterpiece of a vacuum cleaner.