The "best implant" varies depending upon which surgeon you talk to and his/her experience. They all are made according to strict and safe guidelines. Mentor and Allergan both have been in business for several decades and have weathered the implant storms of the 90's and early 2000's, and have both gel and saline implants. Motiva only offers gel implants and Ideal only offers saline. I personally use only Mentor and Allergan because of their long history of safe implants, great company support and stability, and warranty. They both should be big enough to weather the next storm and survive.
there is no study which proves the superiorty of one brand of implant over another. More important is the safety aspect and the only US FDA approved implants are Allergan, Mentor, Sebbin and Ideal implants. This is not to say Motiva implants are unsafe they have been approved to be used in Europe, Japan etc and have received premarket approval in USA. So please consult with your PS to determine which is the best fit for you.