[Eomer of the Rohirrim]

I don't think Pippin was the least useful. Without Pippin's company, Merry may just have given up. Likewise, Gimli and Legolas needed each other to help themselves through the War.
I said Boromir earlier on, and not just because he was a bit alone in the Fellowship. Remember Elrond and Gandalf were wise and deliberated over the selection. I'm sure Boromir was originally going to Minas Tirith and would not have gone to Mordor, so from the start I'm not sure why he was necessary for Elrond's plan.

Of course, everyone had an impact but you have to place yourself in Elrond's shoes at the Council to answer this question.