Assuming you mean as in geological eras? Correct me if I'm wrong there.
As a bird nerd, I'm tempted to say Jurassic only because it was learning about
Archaeopteryx in my initial study about birds that really sparked a fascination for me in evolutionary biology, and
Archaeopteryx originated in the Jurassic period. However, the Cretaceous period was when Avialids really diversified and there are so many spectacular and strange species of bird-like creatures that branched off in that period, so I'd be tempted to say Cretaceous as well...

Good question!
What about you?