THE WORST ammo I've ever shot was some Argentinian-made Concorde .22LR I bought cheap at a LGS back in the '80s. It came in red (hi-speed) and green (standard vel) boxes. The green stuff was okay, just barely. The red stuff was a nightmare. 50% dud rate and when it did fire, it leaded the barrel worse than anything I've ever seen. About half of the duds would fire with a second strike in a different spot; the rest wouldn't. I ended up burning them up in a cheap ERMA-made .22 PPK lookalike simply because that gun was fast and easy to strip for cleaning--every 50 rounds I'd pull the slide off and run a USGI M16 .22 bronze bore brush through it to knock the lead out. I did eventually run through the whole batch of the junk--I seem to recall that I'd bought 2000 rounds of the crap. The cheap ERMA gun was a real trooper through this ordeal.
Second on my list is Remington Golden Bullets. They used to have these cheap at Walmart and about a decade ago I shot about 20K rounds of it through various .22 rifles and handguns. I found that they had a very consistent dud rate of 1% to 2%--i.e., 5 to 10 duds in every box of 550. These duds would not fire no matter how many times you tried different spots on the rim--the problem was they simply had NO PRIMER in them at all. When I figgered this out, I stopped wasting time with second strikes--all duds went straight in the trash. I clearly remember the day I had 100 of these turds fire in a row without incident--in thousands of rounds, this had never happened before and it never happened again. This ammo also produced at least four different sound levels when shooting, but strangely, accuracy was actually pretty darn good and it functioned better than anything else in at least one of my rifles.
Back in the '70s I worked part-time at a sporting goods store and used my employee discount to buy 5000 rounds of Rem Golden Bullets when they came in the useful 100-round plastic packages. Never had any problems with this ammo. In fact I still have some of it and it still shoots just fine, despite having been stored all these years in a garage that gets hot in the summer and very, very cold in the winter. It's MUCH better than the stuff being made today. Go figger.