While there's been a fair few warplanes and a handful of tanks in this thread, I've noticed a lack of anything bigger. So to begin recifying this, I present a weapon so influential that all subsequent examples of the class took her name. I am of course referring to HMS Dreadnought, sixth of her name.

Around the turn of the 20th century, conventional battleship design emphasised the use of a range of gun calibres - 2-4 large calibre guns (typically between 9-12") for long range combat, and a battery of medium calibre (6-9") guns for the higher rate of fire at closer ranges. However, both gunnery testing and the operational experience learned from the Russian-Japanese War had begun to suggest that this paradigm was inefficient, as the bulk of damage was dealt by the largest guns, and the smaller calibre ones made fire control more difficult by increasing the number of shell impacts thus making it harder to determine which shell landed where. The logical conclusion was to switch the design of warship's main batteries over to entirely high calibre weapons, with only small calibre (3-5") secondary weapons to fend off torpedo boats. The Japanese were the first to attempt such a ship, but shortages of suitable weapons meant that the Satsuma had a mixed battery, leaving to opportunity for a truly revolutionary design of warship open to the British.
Under the guidance of Jackie Fisher, First Sea Lord, an accelerated design and construction process led to the new design of ship, christened the Dreadnought (for she feared nothing but God), was laid down in October 1905, and set sail for her sea trials a year later, a truly remarkable turnaround, albeit at the cost of delaying several other shipbuilding projects. When she was formally comissioned in December '06, HMS Dreadnought almost overnight obseleted every other battleship in the world.
Her uniform main battery consisted of 10 12" guns, in 5 twin turrets - and although due to the turret layout, she couldn't bring more that 8 of those guns to bear on a target, that was still twice the weight of fire of the most powerful ships in the Royal Navy prior to her. But firepower wasn't all the Dreadnought offered, as she was also the first capital ship to use steam turbines as a powerplant, instead of the traditional expansion boilers, giving her an unprecedented top speed of 21 knots. She was also larger, and better armoured than anything else afloat.
With every naval power reaslising that their lines of battle were outdated, an unprecedented arms race ensued over the next decade, most famously between Britain and Germany, but all major powers were involved, and the technology advanced so quickly that by the outbreak of WWI the originator herself was badly outclassed. She did still see service during the war, although she missed the one major battleship engagement, Jutland, due to being in port for refits. HMS Dreadnought's one claim to fame in battle came when the German submarine U-29 surfaced in front of her, in order to launch torpedos, only to be promptly rammed by Dreadnought, making her the first battleship to sink a submarine.
Sadly, despite her historical importance, shortly after the war, HMS Dreadnought met the most unfitting end, being sold for scrap, although the name was reused on the first of the UK's nuclear submarines. Her legacy though, was every battleship constructed between her launch, and that of the last one ever built, HMS Vanguard.
Warspite, Bismark, Yamato, Richelieu, Vittorio Veneto, Iowa - dreadnoughts, all of them, and all because of this:
