Verne of course! The father of science fiction, and I think we should consider him the founder of steampunk, or at least its biggest inspiration as well.
I started writing a steampunk-y book before I knew that was a genre. I wanted to write something I would have liked as a kid. I was always looking for stories from another era, and the land of literature is rich with history, of course, and the rules are fantastically elastic. Something set in a certain year could have been written in almost any other year, and the imagined past or present can be manipulated into an infinite number of possible constructions, in a limitless system that should make theoretical physicists' hearts leap with joy. In this wide open wilderness of plausibility, Jules Verne was the greatest pioneer, throwing open the gates of possibility and writing about anything and everything. Plus, he just looks like a nice guy, doesn't he?
Also thanks to him, it's fairly simple to explain steampunk to those who haven't encountered it yet.
"You know, like Jules Verne, if he was writing now about the past."
And in a final note, I named the MC of that first steampunk book Honorine, which I only found out later was also the name of Jules's wife.