Most of all, you're looking for a painting knife with a flexible blade that has a good spring or bounce to it. A painting knife with a narrower blade will bend more than a knife with a wider blade, as shown in the photo.
A very stiff blade is limited in the type of marks you can make with it, and a blade that's too flimsy or floppy is annoying as you can't control it well (and likely won't last). Metal knives generally have far more spring to them than plastic ones.
The handle should be smooth and comfortable to hold. You don't want to be getting splinters from a wooden handle, or have a knife that feels unbalanced. The blade of the knife should be well attached to the handle -- you don't want it to rotate.
That said, I've found there's little to choose between the knives made by the major paint companies, and I've always chosen a painting knife by desired shape rather than anything else.
Buy Direct: Painting knives manufactured by Loew Cornell, Liquitex, Richeson (plastic), Bob Ross (straight).
