If you have to choose only one bag...step one is to decide on color. Most people would say black. For some, a shade of brown or caramel might be preferred. Next, you have to figure out how much stuff you want to carry and whether or not you need/want a shoulder bag only or one that can be carried by the handles.
After buying and selling approx 15 bags, I now only have one bag, a blueberry first. Holds all of my crap, can be carried by the handles or with the shoulder strap, and the color is neutral for MY wardrobe of jeans and tees.
There are so many details we need to know about someone before we can suggest a bag!