It would have to be the ONE knife I would find MOST useful MOST of the time. That would be one I already have...the Spyderco Dragonfly 2. (Now if they made the little light saber with CF and S30V or similar...well now...).
If it HAS to be a knife I don't currently own - then it wouldn't necessarily be based on practicality or common sense or budget or carry restraints. It would be the ONE knife I desire above all others. One I just want. Salivate over. NEED. Ok...it's actually a very simple knife. Nothing fancy or crazy expensive. Maybe $150. It's an ML Kephardt 4 1/4", 1/8 inch thick, 0-1 spear point camp knife, satin blade, with an antiqued osage handle. Dunno why. I just WANT it so bad. *shrug*