First of all, Staraptor defeated Candice's Medicham - he got destroyed by Candice's Abomasnow. And Ash recalled Staraptor because of how much of disadvantage he was against Gastrodon - you can't say if Gastrodon didn't know Counter Shield, he would have won. There's no basis behind that.
Secondly, Swellow was more of a beast. He defeated Winona's Swellow, Katie's Venomoth and Scizor, future champion Tyson's Hariyama and Donphan, Frontier Brain Spenser's Venusaur, and Frontier Brain Tucker's Arcanine/Swampert (alongside Corphish). As Swellow, he always got at least a win/tie before he got KO'd with the exception being Latios.
If you're going to be impressed by Staraptor's resiliency because of how did he against Paul's Ice type (and Weavile was hardly was one of his top caliber Pokemon), then Swellow has easily demonstrated much greater feats of endurance (Thunder Armor, super-effective Donphan Rollouts, entire Tucker battle).
I'm shaking my head at all these people who are voting for Staraptor, who should have zero basis for winning this poll. Cool moveset = / = strongest. Staraptor's resume < Swellow's resume.