The axilla gives the best control over the shape of the cleavage and is the site least likely to form a keloid - a thick & sometimes dark, lumpy scar.
Beneath the Breast
The inframammary fold is the site most likely to form a keloid, but is the only route for large or stiff silicone implants.
The areola usually leaves a nice scar, but is the route most likely to result in infection because skin bacteria live in the milk ducts.
The umbilicus, like the axilla, keeps scars off the breast, but gives less control over the cleavage.
Choosing a proper implant size is not an exact science. Your breast measurements and texture will determine how much extra volume you can carry without overstressing your body, but you will find cosmetic improvement over a range of different volumes. There is more than one "right size" that will enhance your appearance.
Breast augmentation is not an emergency. Take the time to consider different sizes. If you're still not sure, you can buy yourself a few bras in the sizes that you're considering, stuff them, and spend a few days trying out each size.