！！
Bwahahaha! That's hilarious!
Literal
Wahahaha! That [subject-が] masterpiece [copula-だ]!
わはははは is a written-out big, hearty laugh — typical in manga or comics. 傑作 ('masterpiece') is normally a noun for great artistic works, but in casual exclamations it's used ironically/hyperbolically to mean 'that's a good one, that's hilarious.' そりゃ is a casual contraction of それは. A classic villain-laugh or appreciative-mocking response.