Biggest? No. Big? Yes. 1.9 would be the biggest update content-wise based on the magnitude for quite a while.
Although it still baffles me how some people can reduce the update so much into a few words, even though there's quite a lot of evidential changes and new content to go around. I still believe there's a long way ahead, considering Mojang has yet to touch the Wither as stated on the first page of the main 1.9 thread.