The world's largest gold nugget and the largest gold nugget ever found are two different animals...err...nuggets, and they're also different than the worlds largest single mass of gold ever found.
The Holtermann Nugget found at Hill End, New South Wales, Australia in 1872, at 290 kg. was huge, and indeed remains the largest single mass of gold ever discovered, but it can't really be called a "nugget" in my book. We could go into hard rock vs placer, but the essence is that a gold nugget has left the lode at some point and is no longer in the host rock. The Holtermann was what the Aussies call "reef gold" after the quartz reefs sought after by the hard rock miners.
The largest gold nugget ever found was called the "Welcome Stranger" weighing in at 2316 troy ounces (72.04 kg). It was found by John Deason and Richard Oates (pictured below with miners and wives) at Moliagul, Victoria, Australia on February 5th, 1869. While the gold from the Welcome Stranger is still out there, the nugget itself no longer exists. Smelting produced 2268 ozt, 10 dwt 14 grains of gold.
Just below the nugget in the photo you can see "210 lbs." written by hand, but the nugget had to be broken into three pieces in order to weigh it and it's also said some of the miners broke pieces off before it made it to the smelters, so who knows how big it really was...
The largest gold nugget currently in existance is the Hand of Faith on display at the Golden Nugget Casino in Las Vegas, Nevada. Found in 1980 by a man named Kevin Hillier with a metal detector outside his trailer near Wedderburn, Victoria, Australia. I don't know what model of detector he was swinging, but the story goes he got a signal, but thought his detector was acting up, so almost didn't dig! My guess is he was pretty happy he did when he pulled out the 876 troy ounce (27.2 kilograms, 61 pounds 11 ounces) Hand of Faith at about 6 inches. He later sold it to the Golden Nugget Casino for a cool million dollars.
There was another biggie nugget over 2,200 troy ounces back in 1858 called the "Welcome Nugget" (similar, but minus the stranger) and many other huge masses and hard rock deposits, but the Welcome Stranger is still the biggest nugget ever found, and the Hand of Faith the biggest still around today. Of course, I'm still digging, so neither record is safe! Then again, I don't live in Australia...