I think we have three different songs that are potentials that are potential songs she'll be remembered for similar to what other famous artists are remembered for in phases(Ex: MJ is remembered for his Jackson 5 songs like "I want you back", Thriller, Billie Jean, and Beat it. I think Katy will be remembered for the following:
1. I Kissed A Girl - It's Katy's break through song and everyone from different genres loves this song, and it's always memorable.
2. Firework - It's a song with great meaning and everyone remembers this song.
3. California Girls - It's Katy's signature "Candy" look which cemented her status as the candy queen. This will be remembered simply because of the video, similar to why Thriller has been remembered.
Some other notable mentions are Teenage Dream, Roar, and Dark Horse because they're very iconic for her and everyone seems to love Teenage Dream. However, I don't think they're enough to be for her to be remembered for.