It's tough, because Firework will be Katy's signature song most likely. It smashed so hard, then again, BTW I can't see being Gaga's signature song. More like Bad Romance. In any case, what context are we measuring success here? And as far as memorable, I think Firework. I mean, it's still charting more than BTW WW and it was the first of the whole self-inspirational songs. It's certainly a debatable though.
I know people are just gonna complain that Gaga wins the polls all the time but seriously, Born This Way was a ****ing event. An event that doesn't happen very often at all. An event that doesn't just go forgotten.
Firework is definitely Katy's signature hit though, so it's a tricky question.