Like Swampert said, Staraptor is a very good choice. It's got great offensive and Speed stats, and is built to be a user-friendly, straightforward sweeper. =D
I for one, think that Staraptor is also a very top-notch choice! However, in Pokemon, don't choose the "best," choose your "favorite." Here are some quick overviews of other Flying-types:
Chatot:
A Normal / Flying Parrot Pokemon, and my favorite Bird Pokemon (Besides Dodrio, that is :P). It has high Special Attack, rather than Attack, along with Passable Speed and HP. It's downfall is the low Defenses, so make sure you have a way to keep Chatot alive.
Wingull (Evetually to be Pelipper):
Personally, Pelipper is an invaluable companion for an in-game team. It has a fairly useful typing (Although the x4 Electric weakness can be a pain), has good Special Attack, HP, and Special Defense, along with Acceptable Defense, and low Attack and Speed. Its large bulk and resistances help it out, so it's worth checking out!
Zubat (Eventually to be Crobat):
The Bat family picked up Roost and Fly in this Generation, so I consider them "birds" now. :P
They're also good, and can really be used by any player.
Astonish, Leech Life, Bite, Wing Attack, and many other attacks make Zubat have a lot of coverage. They end up having blazingly high Speed,and have fair Attack and Defense. They're balanced for a Flying-Type, so if you're new to the series, this is the way to go.
Drifloon:
Drifloon is not really recommended for in-game. It's got an okay typing (Mostly for the Normal / Ground-type immunities), but it's got terrible bulk. See, it's got high HP, but the Defenses are SO low. This balanced out for average bulk. His Attack / Special Attack are pretty decent, but the Speed is also pretty average. Basically, what you end up with, is a slow, average, fragile sweeper.
All of this being said, I'll lay it out like this:
"VERY EASY" ROUTE: Zubat
"EASY" ROUTE: Starly
"INTERMEDIATE" ROUTE: Wingull / Chatot
"HARD" ROUTE: Drifloon
I've given a HUGE answer, and tried to give as much information as possible. I don't want to be too biased, so from this information, choose the kind of Flying-type you think would be strong AND fun-to-use!
EDIT: I forgot about Tropius. xD
Tropius:
A giant, banana-chinned Grass / Flying type. It resembles a dinosaur! =3
It has a fairly bad movepool, a give-and-take typing (It has a x4 weakness to Ice, but no more Electric weakness, and grass attacks to counter Rock. However, there's also a new Fire and Flying-type weakness too...).
It has average attacking stats, pretty good bulk, and average speed. It's very hard to level up as well.
Which route?
"HARD / VERY HARD" ROUTE.
