Second-half substitute Ince steered home Mathias 'Zanka' Jorgensen's cutback in the first minute of added time to clinch Huddersfield a 1-0 win and spark wild scenes of jubilation at the John Smith's Stadium.
The Terriers' first victory in six Premier League games has given their survival hopes a major boost, lifting them seven points clear of the bottom three.
"It was a very emotional moment for everybody who supports Huddersfield," said Wagner.
"This could be one of the biggest moments in the club's recent history - this moment when Zanka had this little bit of magic and Incey scored the winner."
Ince's last-gasp effort was Huddersfield's only shot on target in a game short of chances, but not on intensity.
"We wanted to make an intense game out of it, an aggressive game," Wagner said.
"We wanted to show that we really want to fight for Premier League survival to give the supporters something to cheer about.
"It will be a really great night tonight in Huddersfield because it was a big step, for sure it was, even if we know the job isn't done.
"People who had much more experience of the Premier League said at the beginning of the season you need 10 wins and it looks like you need these 10 wins.
"Maybe we don't have the best individuals as players, but for sure we are one of the best celebrators."
Wagner defended Ince's contribution this season after the former Derby forward, signed for an undisclosed fee in the summer, took his tally to two in the league this season.
"Today he scored a wonderful and important goal. Every player plays his part in our group and he has done it," Wagner added. "He came on as a sub and he made the difference."
Watford boss Javier Gracia said his players had every right to feel hard done by as their winless league run in the Premier League was stretched to five matches.
"It's the same as the last matches, the same feeling," Gracia said. "The first 15 minutes, I think they dominated the game.
"But after that we felt better and played with more timing. It was an equal game.
"They only shot one time on target and they scored a goal. In extra time again we have lost the chance to get points.
"My players showed desire, spirit and showed that they wanted it, but at this moment the game is being cruel with us."
The Hornets have registered only one point from their last five and have now failed to score in six games on the road under Gracia, who replaced Marco Silva in January.