I'm pretty sure it's A Kind of Magic. That album was heavily discussed as having a Brian-dominance. And I know Freddie only wrote one song on his own on that one.
This is a very good guess and in a way you could say that this answer is close to correct. However, read on.
On
A Kind of Magic, May wrote 2 songs by himself (Who Wants to Live Forever & Gimme the Prize (Kurgan's Theme)), and Mercury wrote 1 song by himself (Princess of the Universe).
However, Mercury also wrote 2 songs with Deacon (Pain Is So Close to Pleasure & Friends Will Be Friends are credited as: Deacon, Mercury).
The rest: 1 song credited as Queen (Taylor), 2 more by Taylor and 1 by Deacon himself. So May
may (haha!) have contributed the most to this album, but Mercury has written on more tracks than May.
If that's not it, it has to be Flash Gordon.
It is!
Flash Gordon is 100% correct.
On this album there are 4 songs by Mercury. There are 6 by May. Plus there is 1 by Mercury, Howard Blake, 1 by May, Taylor and 1 by May, Blake.
For the rest, there are 3 by Taylor and 2 by Deacon.
So,
News of the World and
Flash Gordon have more songs with contributions from May. All other Queen albums released while Mercury was alive (haven't checked later released stuff) have more Mercury than May contributions.
Thanks for playing!