Placement through the belly button claims to eliminate any scarring in the area of the breast. I think that this is a bad choice because it is difficult to impossible to insure that the implant is actually covered by muscle.

Placement through the armpit leaves small but visible scars which  can be seen when you raise your arms. While this may be acceptable to many people, the real question mark with this operation is that at least two papers have shown  that there may be a   higher possibility of permanent loss of feeling in the nipples.

Placing the incision in the fold under the breast is okay, but when lying down, wearing a halter top or a bikini, the scar can be visible. The surrounding skin is all one color and the scars often quite apparent.

I  favor placing implants through an incision made at the lower edge of the areola in most cases. As the implant is placed empty and inflated after putting it in, the incision that you need to make is  small. There is a natural color change where the areola changes to breast skin. This usually hides the  scar  well.

While there is no RIGHT answer to this question, and nobody is WRONG for choosing any of the above incisions, for the above reasons almost all of my breast implants are placed through peri-areolar incisions.

Placing them over the muscle is also not a good idea in my opinion. The muscle coverage helps support them from sagging, hides the edges and leaves a more natural looking breast. They do ride a little high when first placed, but after the pectoralis muscle accommodates, they look better for the rest of your life. This is one area where instant gratification is not  best for long term results. With a little patience, the implants will drop into place and look quite natural in most cases.