The Starly line makes the best flyer in DPPt, as sumwun mentioned. There is a plethora of reasons to use this Pokemon in your playthrough, besides the fact it is a convenient flyer. Some include:
- Starly is available from the very beginning of the game, which means you get a lot of mileage out of it.
- Starly evolves at consistent and opportune times, alongside your starter. This means it is strong and reliable throughout the game.
- It has high Attack and Speed, which makes it ideal for clearing in-game trainers. It also learns Wing Attack very early, which is nice to speed through the early stages of the game.
- Staraptor has a stellar movepool. Close Combat and Brave Bird through level up is excellent, and it makes good use of Return, Fly, U-Turn and Roost TMs.
- Intimidate is a useful ability that means Staraptor isn't a pushover defensively, and can switch into some neutral hits.
Several of the perks above also apply to the Zubat line, which is your next best option. Some reasons you might choose Zubat include its 4x resistance to Gardenia, Maylene and Aaron's Pokemon, its access to Confuse Ray and its better defensive typing. However, Staraptor's balance of power and coverage makes it the best choice.
Murkrow, Drifloon and Wingull are other options, but respectively, they evolve too late, are difficult to access and take too long to become available.