My personal favourite is a 9mm CZ 75 wich I carry with me at all times.(you have to if living in South-Africa

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I think the reason is totally personal and you cant tell anybody wich is better.It is what you are comfortable with shooting.Left and right I can group all 16 my shots in a 4" grouping at 15 meters wich I think is absolutly enough for self defence.I battle to shoot revolvers and this is why my 38 S&W never gets used.
Coiuncidently I am in the market for another handgun for my wife but I'll let her shoot a couple before SHE makes HER decision.I only made a few suggestions but it is up to her.

To get back to the question most of the stats I ever came across showed that a 9mm is the ideal.

If a guy pulls a knife on me I would'nt mind pulling a 454 Casull.Just imagine the look on the batards face