A list made in 2014 by canvassing Welsh writers for their nominations.
This was whittled down to a shortlist of six for public voting: Border Country by Raymond Williams, On the Black Hill by Bruce Chatwin, Shifts by Christopher Meredith, A Toy Epic by Emyr Humphreys, One Moonlit Night / Un Nos Ola Leuad by Caradog Prichard and The Life of Rebecca Jones/O! Tyn y Gorchudd by Angharad Price. Prichard's book was the winner.
The linked article only lists 24 books, though it and other articles say there were 25. I found the 25th by scrolling down the "The Greatest Welsh Novel" Facebook page - they had a link titled 'Greatest Welsh Novel #14: Tywyll Heno by Kate Roberts'. At time of posting (Sept 2020) there isn't an English translation available of Tywyll Heno, but there is supposed to be one in the pipeline from Francesca Rhydderch, titled 'Dark Tonight'.