Is it possible to change the name of this thread to "bird pokemon" instead of "flying-types?" Ash has had a lot of other flying-types in his possession when he travelled through the four regions. If we include pokemon like Gliscor, Charizard, and Butterfree in the mix, it'll be much harder to give credit to Ash's "bird pokemon" since they were used less often in battle. For the purpose of the OP, I'll only include the pokemon that were designated in the contest.
Memorable: The original series always had my best regards. Pidgeotto was clearly the most memorable of the five birds. Even though it was used less often in battle compared to the other birds, Pidgeotto was seen in many Kanto episodes.
Favorite: Noctowl is clearly the most unique of the five birds. Not only is it an alternately colored member of its species, but Noctowl is very unique amongst most flying-type pokemon Ash has had in his possession thus far. Ash has not obtained Psychic-type pokemon yet, so it is interesting that one of his pokemon can use offensive Psychic-type moves. It is because Noctowl was not treated as the "token bird" that I tend to prefer it over the others. It was such a shame that the writers didn't make this pokemon battle more often.
Most Useful:
Swellow has had the best battling record amongst the five mentioned birds. Even though it won very few battles prior to its evolution, Swellow has proven to be resilient when it is in contact with several types of attacks. Swellow was able to resist Electric-type attacks, such as Pikachu's Thunderbolt, even as a Taillow. Swellow has proven its worth in battle. Up until its battle with Tobias's Latios, Swellow has successfully defeated at least one pokemon in every one of its major battles after its evolution. (The tie with Morrison's Gligar counted as a defeat in my books since the pokemon also fainted.)
Pidgeotto/Pidgeot's strength in battle differentiates depending on the context to which it is used in battles. In some cases, Pidgeotto is easy pickings for TRio's pokemon whenever they get into contact. On the other hand, Pidgeotto has had a fair share of victories prior to its evolution and release. It's very difficult to gauge Pidgeotto's strength as it hardly battled prior to its evolution. It's safe to say that Pidgeot has improved a lot in speed and strength after it evolved, as it was able to defeat a Fearow that it was unable to contend with as a Pidgeotto. Still, we have not seen this bird since the end of the Kanto Saga so we cannot hope to gauge its power.
The same could also apply in Noctowl's case, except Noctowl has been used in a fair amount of battles. Even though it is smaller than other members of its species, Noctowl is able to compensate for its smaller size through its intelligence and resourcefulness. Unlike the other birds in Ash's possession, Noctowl has access to Psychic-type moves like Confusion, Hypnosis, and Extrasensory. It also has access to Foresight, an interesting move that allows Normal and Fighting-type moves to attack Ghost-types. Noctowl was able to use powerful Flying-type attacks like Air Slash and Sky Attack in Ash's battle against Conway in the Sinnoh League. Even though Noctowl has a lot of variety to its moveset, this pokemon is very fragile. Noctowl was knocked out by Conway's Dusknoir's Thunderpunch after Dusknoir's Trick Room was activated. Noctowl also failed to defeat Harrison's Steelix and Clair's Kingdra in Johto. Noctowl has a lot of potential, but its presence is underwhelming because it was used in very few battles.
Ash's Staraptor was very underwhelming in its earlier stages. Starly evolved into a Staravia and it learned moves like Aerial Ace very early into DP, but the pokemon still struggled to defeat Grass-type pokemon like Gardenia's Turtwig. Staravia also faired poorly against its opponents in the Hearthome Tag Battle. Staravia has proven its worth after it learned Brave Bird in Veilstone City, and its subsequent victories over Gym Leader Maylene's Meditite and Machoke in the next episode. Staravia was horribly underused compared to Ash's five other pokemon at the time. Staravia got a kickstart to its development once it evolved into a Staraptor. After its evolution, Staraptor became a loose "powerhouse." Staraptor defeated the likes of Paul's Weavile, Nando's Roserade, and Candice's Medicham. However, Staraptor has also suffered a streak of losses in nearly the same fashion (i.e. Nando's Armaldo, Paul's Ursaring, Paul's Drapion, Candice's Abomasnow). Staraptor can be considered a bulkier, physical version of Noctowl. Both pokemon have a lot of potential as final stage flying-type pokemon, but have suffered massive losses. That's not to say that they aren't useful, but their track records are hit-miss.
Tranquill was recently obtained by the trainer, and it hardly competed in battle. Similar to Staraptor, Tranquill has started out as a first-stage bird pokemon that evolved fairly early into the series. Tranquill has not battled any serious opponents since her evolution. Most of her use thus far was battling against TRio. In those cases, Tranquill has rarely, if ever, defeated TRio's Yamask and Woobat. As a Pidove, Tranquill lost many battles that were resolved very quickly. Pidove lost against Jessie's Woobat and Trip's Frillish only mere seconds after they were pitted in battles. Tranquill may become stronger as the series progresses, but right now she is not as useful as some of the other birds in Ash's possession.
Swellow is the most useful of Ash's birds because of its near perfect battling record. Staraptor and Noctowl has suffered a lot of losses in addition to their victories. Pidgeotto and Tranquill were almost never used in battle.
