Hitchin's Self-duality paper.

In this paper, Hitchin constructs a hyper-kähler structure on the moduli space of flat irreducible connections by identifying them with Higgs pairs (holomorphic bundle together with a holomorphic endomorphism valued 1-form).

Initially, the construction was done for compact Riemann surfaces and bundles of rank 2, but generalize to Kähler manifolds, to punctured surfaces (parabolic structures) and to higher rank respectively pricipal bundles.

The paper is not about proving one single theorem (he also introduces the integrable system nowadays know as the Hitchin system), I hope you can count this as an answer, as the subject of Higgs fields is still developing with many interesting connections to other subjects like mathematical physics or geometric langlands.