Pit might be able to out speed Samus and control the stage, however the Zair controls Pit. SH to Zair whenever you have the opportunity. And I don't just mean whenever you feel like tossing it in. I mean make the Zair your main approach, attack, defense, edgeguard, edgehog, and recovery tool. This attack pwns Pit and while it won't kill him (and that's a bold-faced lie, it will Kill him on some stages) Samus has a few other tricks.
Full Jump Homing, Missile Cancel Homing, Full Jump Homing, Missile Cancel Homing, ect... That will flood the skies in Pit's general area with missiles. While this doesn't seem like a big deal it slows him down and leaves him open for, you guessed it, a Zair to the face.
If he's in his shield, F-Smash through it. As Jasona has taught me the painful lesson that the Mirror Shield is useless against opponents who are right up in your face, I can tell you first hand how easily Pit is killed while he thinks he's behind an impenetrable shield. -_-
When Pit is beneath the stage use a Charge Blast or a Zair to try to knock him out of his WoI. Doing so will lead to his immediate death.
The D-Tilt is your best Kill move against Pit as it's fast, sends people very high, and takes away Pit's recovery.
In the end there are many different types of Pits out there. Their are very few who are any good to be honest. To understand Pit a little better come back to the Pit forums and study up. There's a LOT more to him then most people realize.
And besides, the Pit forums are much better then the Samus forums. ^_^ The Pit forums has carnival games!