At first, I would say Talonflame, as you can get it relatively early. But since it overlaps with Charizard, here are some other options.
Hawlucha is simply fantastic in any game that it is in. In XY, you can get it on Route 10, which is relatively early. It also gets STAB Close Combat and Brave Bird.
Dodrio is an interesting option, but you can definitely make it dangerous in the right hands. Doduo is found early on Route 5, with a low chance of spawning. With 110 base Attack and Speed, it can do a lot of damage to a lot of Pokemon. Not the best pick, but a good one at worst. It also has good moves like Roost, Fly, and Tri-Attack, although Hawlucha does it better.
Crobat is good, with moves like Toxic, Poison Fang, and high Speed and Attack. The biggest downsides are its lackluster bulk and friendship evolution.
Aerodactyl is a great team member, with its biggest weaknesses being its inaccurate Rock moves. However, if it Mega Evolves, it will destroy most anything in Kalos. (Thanks, sumwun)
Gliscor is a good Flying type and a great Ground type. It can counter Electric types that the others are weak to, and can fire off STAB Earthquakes.
Other options are good in the late game, like Noivern and Salamence, but it all really depends on what your personal preference is and how far you are in the game. Hope this helped!
Edit: Not a Flying type, but Flygon gets Flying coverage and is a good Pokemon in general
