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?