Most of you probably heard that Fox falls the fastest early on, but Falco and Captain Falcon exceed him after some point.
To give you a quick understanding of how falling speed over time works for all the characters, I created this diagram:



This falling speed is applied whenever you are airborne. This is why light fastfallers like Fox and Falco die from the top blast zone so late – falling speed decreases knockback during the entire hitstun and beyond.

This diagram does not account for:
Related discussions:
http://www.reddit.com/r/SSBM/comments/2l7i5j/falling_speeds_in_ssbm/
http://www.reddit.com/r/SSBM/comments/2lddfy/falling_speed_comparison_over_time_cunninghams/