A person's answer to this is largely determined by their political point of view.
Conservatives are likely to believe that the 2nd Amendment is the one that is least closely followed today. They would argue that there are way too many laws that take away from people's right to bear arms.
Other conservatives would say that the amendment that is really ignored is the 10th Amendment. This is an amendment that, conservatives feel, gives the state governments much more power than they actually have in our current system.
Liberals, by contrast, might also believe that the 2nd Amendment is not followed closely, but for different reasons. They might say that it is meant to protect only the right to have weapons within the context of a militia, not the right to have personal firearms.