I use a Leonard garden knife. It's great for digging holes, sawing through sod, cutting through small roots, prying up rocks, opening bags, cutting twine, etc. The serrated edge is freaking sharp, and the eye-wrenching orange handle is actually helpful in finding it after you set it down.
I really like the Dutch-made DeWit gardening tools ... good quality and comfortable to use. My "must have" tools are a hori-hori (a.k.a. soil knife), a good pair of bypass pruners, and a sturdy serrated garden trowel. I have an absurd number of garden tools, but I mostly use those three.