It's not really an easy question.
It's not fair to discount Pidgeot since we have no idea of what it's skills are these days. We could find out it now knows Aerial Ace, Steel Wing, Double Team, Mud Slap, Roost, or a slew of new techniques. When it was a Pidgeotto, it was very reliable, and saw a lot of use. I don't want to forget about all that experience.
I honestly don't see why Noctowl's getting the shaft. It was Ash's most intellient Pokemon, and we got to see it was sarcastic too, or at least had a sense of humor. As a Flying type, it's unorthodox attacks set it apart from the rest of Ash's birds, and while not seeing much battle use, it proved very resourceful and reliable outside them. I personally think it has the most interesting personality of all his birds.
Now, Swellow is obviously the natural choice. I don't have to recount its amazing battle skills and impressive endurance in the face of its Electric weakness. I also think its personality shown through a decent amount. It is seriously strong, and it saw much use. I would have to choose Swellow as the best bird.
Staravia has done some impressive things, so far. I have to reserve a full judgement until we see the Sinnoh League, I think. There's not a lot to go on as of yet, including some more personality screentime, but I like it so far.