Internet's BestClassic Utility Knife
Basically, wherever scissors won't do, a utility knife is essential. And the way a utility knife is made also makes it the safest and most convenient tool for all your cutting needs; the blade easily slides in and out of the handle of the knife for safekeeping when you're not using it.
At around $10 per pair of these reliable knives, you might as well buy a couple. You'll need them!