A sharp chef's knife (Victorinix is well rated, affordable, and easy to sharpen). If you want to be able to work horse through tubers a sharp nakiri (Japanese style vegetable knife) or a Cai Dao (Chinese style vegetable cleaver) or even a western style cleaver would do worlds of wonder over any vegetable slicer.

An BIFL alternative to a knife is a mandolin with changeable blades. However, these require far more safety precautions than a knife and it's reccomened to wear a slice proof kitchen glove (a multi-use consumable item).