I agree with everything that has been said so far. I voted for Firework, since that is really the song that transformed her career into what it is now. And, as others have said, it's an inspirational song that has helped a lot of people, and it has potential to be played for 4th of July. Uptempo inspirational songs seem to be what Katy does best, and Firework being a perfect example of that.
I didn't include E.T. or Dark Horse in the poll, since even though they are both huge hits and both great songs, they aren't really typical of the type of song Katy is best known for.
I'm surprised that nobody at all has picked Teenage Dream. It is the title track of the album that she'll likely be best known for. And, for most people, it was the first somewhat serious song of hers that they heard, as well as the first to really show off the beauty of her voice. I know that One of the Boys has other songs that fit those criteria, including the single Thinking of You, but I had never heard any of them (not even Thinking of You) at the time that I first heard Teenage Dream, and I suspect that is true of most people.