I believe too many people have it in their head that a best truly exists, but it simply doesn't, any more than there is one best car for everyone, or one best job for everyone, or one best place to live for everyone.  It's a question for which no answer exists. 

When we ask that question we are either unaware of the myriad of different options or we are hoping the person we ask will trip over his own feet and fall flat on his face.

So, yes, it's a question that can only be answered with more questions.  Who is asking the question?  What kind of person are they?  What are their needs, their background, how much training do they have, will they get?  Will they practice?

All that said, I do believe we can offer some generalities.

An adult man or woman who is not infirm can handle a service caliber handgun, and should always choose that for concealed carry.  The reason is because the types and number of attackers may well call for the ability to shut down the physical plant.  We shouldn't expect to scare our attackers, or to shoot them "a little" and hope it wards them off.  I have several police officer friends who tell me bad guys today are more aggressive and less reactive to negative stimulus than ever before.  Frankly Jerry always said the same thing. 

Since we know that any handgun cartridge is marginal for shutting down the physical plant when all else fails, we really need to carry the most effective round we can, allowing for shootability.  We must be able to handle the recoil, we must be able to bring the handgun back on target quickly.

9mm and .45ACP are not the only options.  Several cartridges which produce around 400 pound feet of energy are viable options.  I'm not an energy nut but if we look just at velocity you let in some of these ultra-light, 2000fps rounds that have not been proven viable by any police department or military against human targets (5.7x28, .22 Magnum).  And even just balanced velocity/energy isn't the last word, as high bullet weight/high momentum has been a deadly combination in history  (think .45 Colt/.44 Special).  One must balance the shooter to what he or she prefers and can handle.  New shooters definitely need good advice, although good advice is not always easy to find, as we have placed concealability over performance in some cases, and sales clerks and even instructors tend to parrot what they hear from "higher authorities."  If that wasn't the case we wouldn't be advising so many healthy adults to carry .32s or .380s when they may have to use it against a drug-crazed attacker who will absorb every bullet that little gun holds and kill them anyway.  The attacker might die seconds, minutes, or days later, but little consolation.  Or the .380 carrier may be attacked by a gang of punks--this seems to be a greater concern to me for those as they age and began to appear more like "targets."  Or for women.  Same concern.