I googled this before and it seems that there are flat seeds from 2011 - but no recent ones. All of the seeds are now changed. Currently, what is the flattest seed out there?
Epic minecraft OG
If you want a completely flat world you can make a Superflat world.
If you want a normal world that is flat around spawn you could try
(Without the quotes of course.)
Map and video in this thread:
Thank you for your brave expedition.
(I understand that you can make a superflat) but I want a legit default world. I rate this seed 7/10