Although the 17th is a close second.
You don't like people voting for their own senator?
There exists the House of Representatives to represent the interests of the people. While the erosion of federalism in the United States began long before 1913, the elimination of the states' representation in the federal government certainly contributed to the trend.
I can understand the argument. One chamber represents the people while the other represents the states. I don't think that just because the senators are now elected by the general population instead of state legislatures there is less representation of states interest. People of a state are still going to elect someone that represents the state, are they not? And how did the way senators did their job change after 1913?
The Senate should be a body that is above the whims of popular opinion; having them be directly elected by the people of each state defeats that purpose of having a branch of the legislature not bound to popular will. As it stands now, the Senate is essentially a redundancy. Of course the popular branch of the legislature is also corrupted, having over twenty times the ratio of population to representative as originally intended. Thus, I would not be opposed to repealing the 17th Amendment on condition of adjusting the average population of congressional districts back to 30,000. Hopefully more people would then come to a reasonable conclusion of how to deal with a legislature of ~6,000 people without once again raising the population to legislature ratio.