You'll need the biggest caliber you can control and are comfortable carrying. Shoot some of your choices and see how well they fit you.
I don't think anyone can say a particular gun is best, because people are so different. There are advantages and disadvantages to revolvers and semi-autos. They can often be resolved by your particular situation, if you answer enough questions.
I have five different carry guns that I carry depending on the weather, dress, etc. I have a full-size 1911, a 3" 1911 (both are 45 ACP), a 32 H&R Smith & Wesson revolver, a M&P in 9mm, and a Kel-Tec P3AT. The little guns get carried more in the summer as they are more concealable. I only carry the full-size 1911 if I'm in a situation where I could reasonably expect trouble. The M&P stays at work.
What do you enjoy shooting? If it can be carried, that'd be the starting point for me.