An 8-10 inch mid level chef knife. One with a single bevel made of quality steel that can hold an edge will make a world of difference. You haven't lived until you've cut paper thin potato slices like butter
A Victorinox chefs knife is only around $30 and I like it more than my expensive German knives. Between the two, the Victorinox feels like a surgical instrument and the Wusthof feels like a splitting maul