Who first proved the original classification of compact connected surfaces?
There are many different proofs, for example Seifert and Threlfall's proof (1934) or Conway's proof (1992). However, I suspect there are many more, for example in this article it states the theorem was known as early as the 1860s.
From https://www.seas.upenn.edu/~jean/surfclass-talk2.pdf (the 13th slide):
Thus it should be Brahana who first proved the classification of compact connected surfaces rigorously.
Möbius' original publication: A. F. Möbius. Theorie der elementaren Verwandschaften. Abhandlung Sächsische Gesellschaft der Wissenschaften , 1863
Jordan's original publication: C. Jordan. Sur la déformation des surfaces. Journal de Mathématique 11:105–109, 1866.