The best knife in a Defensive or a Survival situation is the one you have with you at the time, however, certain designs and knives may be better than others. Survival Knives are generally 5.5" or longer, 3/16" or thicker blade to handle abuse such as clearing brush, chopping, processing game and can be field sharpened with basic tools. They may also be used defensively but were not specifically designed for that purpose. Some have pouches on the sheath or a hollow handle to store some survival items.
Defensive Knives are a more single purpose design. They can still be used for cutting but are not designed for heavy use and abuse. The blades are narrow coming to a spear point such that they can fit between the ribs on 2 leg and 4 leg attackers. They may have both edges sharpened at least part way and are often referred to as daggers or boot knives. Blades generally range between 3" and 6" in length although Special Forces groups also favored a longer 7" blade like the Gerber Mark II's and it's many clones like the Valors.
Bowies, in my opinion, they are a little of both. They usually average over 5" and often have a false edge in a clip or spear point which makes them also offensive.