The old "industrial" clay recipe from IC1, in IC2. I think it was one gravel, one sand, one dirt and one water bucket for one clay block. Of course, a water cell could also be used.
And before you do the "there's more clay in the world now" argument, please think about the fact that clay is still only available in rivers and oceans, from where it is quite annoying to extract. Alternatively, the recipe could be set to also require a block of clay, and then return two blocks?
EDIT: If you re-ad this, maybe also consider re-adding the wrench of almighty banhammoring +9001? IT WAS SO AWESOME! I know, i know, we have creative, but it could be usefull for insta-breaking blocks in survival, and being totally awesome.