X-acto #1 knife
It’s hard to go wrong with a classic. X-acto knives aren't just useful for the cutting of paper, but are flexible, amazingly useful tools in general. A lot of the Bolt team members keep one handy at our desks for occasional poking, prodding and scraping of prototypes. The handles are simple but last forever and the blades are cheap in bulk, so you have no excuse not to swap them out for a fresh one once they chip.