Here's a knife that will handle all cutting tasks (except food prep) realistically encountered in an urban environment:

I can highly recommend this Olfa H-1. It's a tank, and my go-to for construction work. I have had thinner, cheaper utility knives snap off or unlock under extreme pressure (the only time I needed stitches!). The "extra heavy duty" blades are thicker and wider, and well supported by the handle.

The blades are readily available. I also resharpen the tip when working, so each section of the blade lasts a very long time.

Cost: under $15 on Amazon. Every majory hardware chain carries these as well.