Unless you have extraordinary training, no knife is suitable for self defense.
This is patently incorrect. Using a knife effectively requires very little training. Yes, there are entire arts devoted to their use, but at it's root, using a knife for self defense boils down to "hold it by the dull end and hit them with the sharp end".
Without taking anything away from the arts devoted to knife fighting, I honestly think most people can learn to use a knife effectively in a few hours. It may not be elegant (neither is basic empty hand self-defense) and it may not be effective against an opponent with more training (as is true of [insert empty hand system here]), but it will be effective.