If you know me, you know I love the early bird Pokémon and all other flying types, so it's really difficult to choose. If I had to pick, however, I'd have to say either the Taillow line or the Starly line, with the Fletchling and Pikipek lines coming in at a close second.
Why? Well, the first game I ever played was Pokémon Diamond; a borrowed game from my cousin (sorry for deleting your save file, man) Starly was one of the first Pokémon I encountered (besides bidoof), and I even did a nuzkocke run on Pearl with one (RIP Brittany) and I have since grown attached to them.
The Taillow line is one of my favourites for another reason entirely. When I was younger and still lived at my previous house, I used to raise butterflies. No, not the ones from those fancy kits. My neighbors had a problem with caterpillars eating their parsley, so Past Riverrunner swooped in and decided to raise them on her own within a large plastic jug. The resulting butterflies were called
Black Swallowtails (and they are still today and will always be my favourite species of butterfly :3). The taillow line, based off of the real-life swallow (bird) reminds me a lot of them and that's why I love that line in particular.
As for the runner-ups, they're on the list because of my fascination with Phoenixes (or any mythical creature really) and because Trumbeak is just an awesome Pokémon
toucan carry its own weight on my team (despite the whole woodpecker to toucan debate).
That...
That's it.
There isn't a long answer for those last two, I'm just a dork.