Buy a quality knife as suggested by others (Case, Gerber, Schrade, Marble, Kershaw, Puma - etc). Don't get one with a blade that is too long and avoid "Survival Knives". A 4" blade with a "drop point and a slightly curved cutting edge is about right for deer size animals (larger blades up to 6" can be useful for larger animals such as elk and moose). The drop point will help avoid puncturing organs and the curved cutting edge is better for skinning. Get one with a design (either handle shape or guard) that will prevent your hand from sliding from the handle to the blade. I prefer one with an adequate "choil" (a dull space on the cutting edge of the blade between the guard and the sharp cutting edge so I can put a finger on both sides of the guard. When cutting inside the deer (to loosen the entrails where you can't see) lay your index finger along the top edge and use your finger to guide the knife.
The small saw blade on a "multi-tool" can be used for limited bone sawing (such as the pelvis and sternum) and I once even used the small knife on one for field dressing.
I've also used the Outdoor Edge "Game Skinner" knife successfully to field dress and skin (it is an excellent skinning knife), however it's shape is less useful for other hunting relating cutting, so I would not recommend it as unless another knife is also carried.
Fifty years of big game hunting experience during which I've used a variety of knives from pocket knives to 6" blades to field dress (and in some cases debone) many deer size and larger animals.