California Gurls is a classic because it's a summer staple.
Literally that song's airplay surges during the summer.
+ it was big on a global scale too, it went top five in every major market sans Japan(top ten and certified Gold).
IKAG is a classic because EVERYONE knows the lyrics. Plus it's still heavily attached to Katy's name.
I mean that song went #1 in 18 countries.
Certified pretty much everywhere too.
I'm sticking with her having five classics.
Which is crazy considering she's only had three albums.

I don't know how some people deny the fact that she's at the top of the pop hierarchy with Tay and Bey.
