'Thinking Of You' is one of Katy's best songs and probably one of the only ones from her discography I feel actually deserved to go #1.. really a shame that it didn't, but then again I don't really think the GP uses Katy for her serious, heartfelt love songs, so..
It's like having to choose between the death penalty or life in jail. "Thinking of You" sucks a little less, but this thread has served to help me realize the fact I can't really name a Katy ballad I enjoy off the top of my head.