I think everybody's first sidearm should be a revolver. Much easier to learn good shooting habits, single action first then double action. Wait to make a pistol your second gun.
Good advice, but not always applicable. New shooters can become frustrated quite easily, and revolvers (with their heavy trigger pull) can add to this frustration..assuming the use of DA or DAO. Now, if they shoot a DA/SA in "SA" mode all of the time, perhaps they will not become as frustrated. However, Im not sure if I agree that one should begin with SA as this can become somewhat of a crutch. Believe me, I know as this is how I started out. This led to a slight resentment/inability to compensate for DA which took me quite a long time to overcome. This may not seem like a big deal, but for those who may one day CC or keep a weapon around for HD...it could be. Besides, while DA practice can certainly be a bit more to deal with, if just starting out, I think that, in a way, it almost "forces" one to pay much closer attention to the core mechanics of shooting vs. becoming too reliant upon a light trigger pull to compensate for other mistakes. Now, if one insists on having DA/SA capability, I think the best option would be a semi-auto (depending on model, of course) as it would start out with DA for the first shot and then go SA from there on. With the revolver, a hammer pull is required after every shot (for SA). Not very effecient IMHO.
Either way, I think beginning with a 9mm is a decent bet. Its relatively cheap to fire, does not offer too much recoil and after all is said and done, can make for a fine CCW or HD weapon. The trigger pulls also seems, on average, to be a bit easier to cope with vs. many revolvers in stock form. Now, in terms of going with "traditional" style SA/DA or DAO (or DAK if speaking of the SIG), I cant say.
However, for instance, my wife started out with a DA/SA Sig P239 9mm and while she shot it very, very well, she opted for the DAK version even though she found this a bit more difficult to hit shoot. Reason being, given her desire to own a DAO revolver one day, she felt it best to have a semi-auto offering action closer to that of the revolver...for consistency/practice concerns.
Either way, revolver or semi-auto. I think whats "best" is what feels best/shoots best within the hands of a beginner (which is, of course, very subjective as we are not all made/do not think the same). All the advice in the world wont mean squat if the user just does not "feel it" when shooting a particular caliber or platform. Best thing is to find what "feels" best and shoot, shoot and shoot some more. Almost any weapon can be learned with enough practice. Now, one may find that, upon purchasing their first weapon, they later come across a different caliber/make/model which seems even better than their original choice. Thats OK...Just take what you have learned and apply it to the new weapon. Calibers/models/platforms may change, but the core mechanics of shooting apply to all of them. Point is, above all else, whatever you choose...just use your head, dont be in a rush, enjoy the process and shoot the gun...often!
Now for me, in terms of brand, I like the Sigs and the XDs. The Sigs are generally the best shooters (in my hands at least), but are quite expensive and as of late, their reliability/quality has become suspect. But, I have yet to personally encounter any such issues and therefore, still consider myself a Sig fan. Regardless, in terms of overall quality, shootability, feel, features (all for a small price), I find the Springfield Armory XD hard to beat. Sure, they dont feel as sweet as the Sigs, but these guns are tanks....and accurate ones at that. If I were wanting a revolver, Id probably go with the Ruger GP-100.