What makes a good "whittler” (i.e: wood carving/sculpture knive) IMO is a combination of how comfortable the knife is to use and how well the blade takes and holds an edge. Slipjoint pattern knives are the historical, traditional go-to blades for “whittling.” Tradition notwithstanding, dedicated fixed blade knives designed specifically for wood carving are far more comfortable to use and are far better designed for effective, detailed carving. Those knives have large, well-shaped, ergo handles (and usually small blades) and they’re a lot more comfortable to use than slipjoint pattern knives. The carving knives offered by Pfeil, Flexcut and Helvie are good examples but there are lots of other good ones available.
You could probably do most carving projects with most knives, but if you want to choose a knife for wood carving your first decision has to be whether to go with tradition (a slipjoint pattern knife) or function (a fixed blade dedicated carving knife). If your carving activity will be limited, almost anything will do. If you’re going to tackle larger or more serious carving projects, you may find the traditional slipjoints are uncomfortable for extended use and that the dedicated fixed blade carving knives are the way to go.
Good luck with whichever way you decide to go.