Make, model and caliber.
OP.....what's your budget?
Single stack - SW Shield, Springfield XDS, Glock 43. Those are available in 9mm and will be easy and comfortable to carry. Only downside is number of rounds they hold. You can go with a double stack gun like a glock 26 or 19 but they wont be as comfortable. I carry a glock 19 fine but its still something to note.
Different for every individual. Every individual has their own criteria upon which to base the decision.Make, model and caliber.