From OOTB, it is probably Thinking Of You, because I once found a video of one of Katy's earlier live performances from which I could copy all her chords in it; they're not the ones which are in the books, and every time I play it now it still feels like something incredibly special to me. Unfortunately, the contents of the lyrics are not far from my own experience, even with my first girlfriend ever I was kind of thinking of a different woman, which didn't feel quite right.
From TD it's Firework for me, I just love both hearing and singing it, it's like taking your vitamin D pills on a dark night in winter, and what it does for me makes it my favorite of all the albums together.
Hard to decide between Dark Horse and Unconditionally on Prism, but if I have to I'll pick Unconditionally because it's such a beautiful song to sing, and I remember how it meant so much to Katy from the beginning which is also a factor. This week I want to sing 3 songs in a place somewhere THU evening, and the only one I've even definitely decided on yet is Unconditionally.
And, maybe Search Me; the easy but totally effective guitar style fascinated me.