There are multiple options, and no one option is best for everyone.
I agree with my colleague below that the umbilical scar is not the best, as it is much more difficult to accurately shape the breast from this distance, and in my opinion it does not make sense to compromise the outcome in the area of interest (the breasts) in order to have any particular scar.
The scar around the nipple (peri-areolar) is a viable alternative, and need not necessarily compromise nipple sensation, but will absolutely require dissection through your breast tissue. This dissection exposes the implant to bacteria in your breast ducts, and some theorize that this exposure may lead to capsular contracture. I think it is important to understand this, given that we do not truly know what causes capsular contracture, and that if it occurs, you will need surgery to correct the problem. It can be a very inconspicuous scar, but if you are among those in whom it doesn't heal perfectly, you will have a scar that is visible when you are out of clothes, in the most central area of your breast.
The armpit scar is an excellent option as well, but as with the belly button, it is (in my opinion) more difficult to carefully and precisely shape the breast reliably from this approach, and I have seen many secondary deformities resulting from this technique (meaning that over time, unfavorable changes in the breast shape occurred). I think that in very carefully selected patients, it is a good option, but it's not for everyone.
The scar under the breast has the disadvantage of being on the breast itself, but on the other hand has a few significant advantages (in my opinion)- because of it's location, it is NEVER seen (unless you want to show it to somebody), even out of clothes. Also, because dissection through the breast tissue is not necessary, theoretically there should be less exposure of the implant to bacteria in your breast ducts. Finally, because of the proximity of the incision to the areas of the pocket that require shaping in order to achieve the most lovely breast shape, the surgery (and getting the outcome you want) is a bit less complicated, lowering risks and increasing odds of satisfaction.
I still use 3 of the 4 incisions mentioned (don't use the belly button), but do think that for the majority of women the scar under the breast is most likely in their long-term best interests.
I would think carefully about the above and what is most important to you, and listen carefully to what your Board Certified Plastic Surgeon says about your options.