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 and can be field sharpened. They may also be used defensively but have not been specifically designed for that purpose. Some have pouches on the sheath or a hollow handle to store some survival item.
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. Often they may have both edges sharpened at least part way and are referred to as daggers or boot knives. Blades generally range between 3" and 6" in length.