What have been decisive weapons?

[Mangudai]

Off the top of my head and in no particular order:

Recurve Bow
Tank
Flint-lock Muskets
Rifled Barrels (as differented from smooth-bore)
Repeating Rifle
Machineguns
Aircraft
Ironclads
HMS Dreadnought
Aircraft Carriers
Corvus

I'd take Flint-lock Muskets, Rifled Barrels, and Aircraft off the list. While these all became important eventually, they were not game changers right off the bat. The flint-lock was about equal to the matchlock except less prone to accidents. The rifle didn't truly trounce the smoothbore until about 100 years after its debut. Because smoothbores reloaded faster (looser fit) and could use buck and ball. Aircraft were not a game changer in WWI.

I would add breach loading rifles. An early example is Abel Straights Lightning Mules Brigade, which trounced vastly superior numbers in Georgia. The Germans soon improved their doctrine by training soldiers to fight on their bellies. They absolutely crushed their opponents in the Franco-Prussian war.

[Kekvit Irae]

For me, a favorite would be the British Mark 1.
It wasn't very decisive on the battlefield, but just the very invention and deployment of the weapon revolutionized combat for the next hundred years, most notably in 1939.

[Ja'chyra]

Clovis spearpoints
The Bow
Iron
Horses
The Gladius
Gunpowder
The rocket
Aircraft
Tanks
Chemical, Biological and nuclear weapons

Although all of these can be worked around with the right tactics. Almost all weapons could be considered to be decisive in some sense of the word but it really depends on how they were used and who by, there'd be no point giving a nuclear bomb to a stone age man. But I think the main thing is how they were used.

[Seamus Fermanagh]

I think I'll side with Robert Heinlein on this issue. There is no decisive weapon -- a weapon is simply a tool.

Weapon systems are secondary. There is, or soon is developed, some counter to any such.

Discipline, training, & experience are the keys. Dangerous people can defeat you with any tool or create the tools they need.

[Uesugi Kenshin]

I think I'll side with Robert Heinlein on this issue. There is no decisive weapon -- a weapon is simply a tool.

Weapon systems are secondary. There is, or soon is developed, some counter to any such.

Discipline, training, & experience are the keys. Dangerous people can defeat you with any tool or create the tools they need.

In the same vein I would then say that truly decisive weapons are either new methods of making war (tactical or strategic), new methods of training for war, and most especially a combination of all three.