In the paper The greatest mathematical paper of all time, by A. J. Coleman (The Mathematical Intelligencer 11 (1989) pp29-38, https://doi.org/10.1007/BF03025189) the author argues that Wilhelm Killing's 1889 paper Die Zusammensetzung der stetigen, endlichen Transformationsgruppen. Zweiter Theil (Mathematische Annalen 33 (1889) pp1-48 https://eudml.org/doc/157397), referred as Z.v.G.II, was the most significant mathematical paper he have read or heard about in fifty years, because:
(1) It was the paradigm for subsequent efforts to classify the possible structures for any mathematical object. Hawkins  documents the fact that Killing's paper was the immediate inspiration for the work of Cartan, Molien, and Maschke on the structure of linear associative algebras which culminated in Wedderburn's theorems. Killing's success was certainly an example which gave Richard Brauer the will to persist in the attempt to classify simple groups.
(2) Weyl's theory of the representation of semi-simple Lie groups would have been impossible without ideas, results, and methods originated by Killing in Z.v.G.II. Weyl's fusion of global and local analysis laid the basis for the work of Harish-Chandra and the flowering of abstract harmonic analysis.
(3) The whole industry of root systems evinced in the writings of I. Macdonald, V. Kac, R. Moody, and others started with Killing. For the latest see .
(4) The Weyl group and the Coxeter transformation are in Z.v.G.II. There they are realized not as orthogonal motions of Euclidean space but as permutations of the roots. In my view, this is the proper way to think of them for general Kac-Moody algebras. Further, the conditions for symmetrisability which play a key role in Kac's book  are given on p. 21 of Z.v.G.II.
(5) It was Killing who discovered the exceptional Lie algebra $E_8$, which apparently is the main hope for saving Super-String Theory -- not that I expect it to be saved!
(6) Roughly one third of the extraordinary work of Elie Cartan was based more or less directly on Z.v.G.II.
Does English translation of Z.v.G.II exist?