What have been decisive weapons?

[The Wizard]

None of those mentioned in the OP were warfare-changing weaponry and therefore don't fit the bill.

I'd say the most influential are the repeating rifle and the machine gun, which gave European (Western) armies their first and only decisive technological advantage over their non-Western rivals, and for a short time (approximately two to three decades) gave them an unparalleled ability at war. That this period coincides with the Scramble for Africa is no surprise.

A trailing third, after those two tied at first, I'd say would be the airplane. And maybe, just maybe, the tank for fourth place.