Hmm... Now this is an interesting topic. The question itself is not interesting, because it's the opening for many a thread long and short, all over the net. But I've never seen the topic given thoughtful treatment.
First off, I don't have a carry permit, so I don't carry (I have had three altercations in the last 30 years, all of which I solved with my hands and feet - Unconscious is usually as good or better than dead). I am considering getting a carry permit because I often travel with $10k-$20k worth of guns, and I'd be a juicy target if my car broke down or I got into an accident.
My own choice would be my AMT 45 Backup. I think 6 rounds of 45 in a SMALL, reliable, good pointing gun is unbeatable if the gun is only ever coming out when you intend to use it.
If, however, I had a job where I expected to have to take a measured response, say a rural cop, security guard, or bouncer, I would want to have the option of using the gun for INTIMIDATION, and would want a full-sized, chrome-plated 1911.
If I were in the LAPD I would want the gun with the greatest number of bullets, so that I could close my eyes and make a lot of noise while I pissed myself.
Now, having said all of that, I DO understand that full time carry has a COMFORT requirement, a RETENTION requirement, and for non-uniform carry, a CONCEALMENT requirement (though you can spot an LAPD detective from a block off because of the hair and body armor).
Did I miss an important criterion? Let's get the criteria fleshed out, and then everybody can justify their carry based on agreed criteria.
Okay? Begin!
So far we have size, intimidation factor, comfort, retention, and concealment. We can take reliability and shootability as gimme's, since nobody would argue that they are negotiable criteria.
So in my case, the AMT 45 Backup is the ultimate in size and concealment, and would rank pretty darn high in comfort if it were to be an all day carry. It ranks pretty low on the intimidation scale, in spite of the damn big one-way tunnel to hell - folks are as likely to think I am offering them a hit on my crack pipe as preparing to shoot them. You could walk all over town with it in your hand and nobody would notice it.