For me it would have to be a national ban on firearms. Likely handguns or rifles as I know the importance of both in Revolution.
I'll play - one problem I have with gun list revolution threads is that they typically focus on revolution only based on gun confiscation schemes. I don't remember other violations of liberty that would raise the flag.
I think that violations of the other items of the BOR are equally important. Closing down a free press or instituting a state religion or other scenarios are equally important. In fact, I sometimes see gun lists (not here typically)proposing horrendous rights violations but folks are OK with that if they could keep their guns and the groups getting screwed don't fit their social views.
When I see folks actually arguing for general liberty protection outside of just opposing gun confiscation, do I take the topic seriously.
The argument is that the 2nd protects all the others but then you don't ever see anyone talking about the 'others' - in fact, some folks are happy with those violations.