Personally, especially for a first pistol, I would not worry about what's the most common round or what pistol is the most common or best. Rent as many pistols in as many different calibers as possible. Stick to any of the common calibers and get what works best for you. If at all possible, avoid buying a model and caliber that you have never fired. Also, keep it's primary use in mind. Will it be for daily carry, home defense, range use, plinking, a little of everything? Your "best" option may vary on it's use.