PRISM is definitely Katy's peak. (so far

)
Music-wise, we have Roar and Dark Horse. Both sold 6m in the US and has 1 billion video views. I believe they will define Katy and become her signature hits (along with Firework, of course) many years down the road.
Then we have the Prismatic World Tour which was the first time she opened in arenas in so many countries.
Her earnings have also increased tremendously since TD, as well as her celebrity status, proven by Roar's release, Super Bowl, etc.