Don't overlook the state laws over the National laws. I'm sure the gun enthusiasts in New Jersey could consider the 1966 Sills act mandating a permit system in order to buy a firearm worse than the 1934 GCA.
Those in NY might consider the 1911 Sullivan Act the worst.
Perhaps the gun enthusiasts in Illinois would consider their 1968 FOID (mandating a permit in order to buy and possess a firearms) the worst ever.
I see good arguments for the 1934 GCA being the worse.
I'll go out on the limb and say the 1968 GCA was the worse. No more mail order sales of pistols and rifles. Anything over 50 cal was classified under NFA (unless it had a sporting purpose like shotguns). Importation of firearms was restricted unless they met some kind of arbitrary standard. Interstate sales of firearms were illegal unless one had a FFL, or it was a long gun in a contiguous state. Ammo sales were restricted. 1968 GCA was pretty bad in my opinion.
Edit:Addition
Also positive ID was now required for all sales for the 1968 Gun Control Act with an additional age restriction or 21 years old for handguns and 18 years old for long guns. Because of the importation restrictions, surplus firearms that were formerly widely available was now curtailed.
The Best event for 2A:
The rise of 'shall issue' conceal carry laws across the United States.