Best is a subjective term. It depends on the person being asked.
What is the best car? What is the best sports team? What is the best religion?
Just to provoke some discussion in here, what does everyone think is the best reason for a person to use Linux?
It could be:
Freedom
Low cost
User-friendliness
Useful features in the desktop
Power-user features
Customisation
Security
or anything more specific!
What do you think is the best reason?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic