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I would recommend one very short "paper" by Grothendieck in some IHES publications has defined algebraic de Rham cohomology. (I don't think it maximizes the ratio in question, but it is an interesting one, anyway.)

BTW, it was actually part of a mail to Atiyah. It begins with 3 dots! (Maybe some private conversation was omitted). Of course, sometimes Grothendieck wrote long letters (e.g. his 700-page letter to Quillen "pursuing stacks" or his 50-page letter to Faltings on dessin de enfant)d'enfant).

Also, I think Grothendieck had a (short?) paper with a striking title called "Hodge conjecture is false for trivial reason", in which he pointed out that the integral Hodge conj. is not turetrue, one has to mod out by torsion, i.e. tensored with Q.