X-Acto X3000 Knife
This is my favorite knife to use. The barrel is rubberized for a no-slip grip and it's slightly contoured so it fits nicely in the hand. This particular model also has a handy storage compartment where you can stash a few extra blades. If you'd like to see it in action, check out my Cut Paper Crossover class on Skillshare.
Available here: X-Acto X3000 Knife with Cap
Replacement blades can be found here: Replacement blades