Which brand breast implants is better Mentor, Arion, or Nagor ?

Your questions asks about Mentor, Arion, and Nagor.
Nagor is a British implant and are are not available in the US, nor there are no plans to get it approved by the FDA.
Arion is a French implant and there are also no plans to bring it to the United States.I'm actually surprised that most of the doctors answering this question have taken the stance that the implants are quite similar.  It is true that all are FDA approved and meet very stringent standards.  Yet they are very different in dimension, shape, feel, warranty, corporate support, and perhaps longevity.  These sorts of differences are subtle and nuanced, but those differences should matter to someone researching an elective operation.
Mentor is a longstanding US company that is currently owned by Johnson and Johnson. Their implants are safe and popular, however surgeons who are attuned to the developments in breast implants will note that Mentor has not innovated or launched a new implant in years.  They have reduced their sales force, minimized their corporate support at plastic surgery meetings, and so the commitment of JNJ to this space in the future is being questioned by many. Rumors abound that the company is for sale.  Whether or not that it is true, it does attest to the notion that they are falling behind the others.  Their round products only come with one level of cohesivity and one choice of fill to shell ratio.  The other manufacturers all have options for different cohesivity and fills.  Their teardrop implant is not well-shaped and is inadequately cohesive to maintain the intended shape.  Some data shows their silicone implants to have the highest rupture rates. To be clear, many thoughtful surgeons prefer them and achieve excellent outcomes.  But I stand by my comments.
Sientra is a relatively new implant manufacturer and they have an exciting portfolio of products.  They have taken the responsible position of only selling to qualified, board-certified surgeons.  So you know if you get a Sientra implant that you will be connected with a properly trained doctor.  However their former manufacturing facility in Brazil burned down over a year ago and they still haven't started manufacturing again at a new facility. So at the current time the only available Sientra implants are from the pre-fire inventory. Though I used a lot of these before the fire,  I'm personally not comfortable with that situation. However many excelelnt surgeons are using them and the implants are indeed terrific.
Allergan is now the leader and they have innovated and developed the most of any of the manufacturers.  Their round implants come in three levels of cohesivity and at least two degrees of cohesivity.   These are very important concerns for balancing shape, feel, resistance to folds, and probably longevity.  Their teardrop implants have the best shape of any and are cohesive enough to preserve that shape.  At this point they have the most complete product line and the knowledgeable surgeon can choose an implant that very specifically matches the needs of any specific patient.  
I have extensive experience with all three and i can say with great confidence that at this time Allergan is the best choice because of its range of products.
As a matter of disclosure, I was a paid consultant for all three manufacturers at one time or another, but at this time i have no relationship with any of these three.  Though at this  I am consulting to a new company called Motiva, which is currently not available in the USA.

There could be a conflict of interest in this question, still I do not work or have preference for any brand.. There are few FDA approved implants: Allergan, Mentor, and Sientra implants. In all other countries including Europe they are other widely used brands. And thus not using FDA approved implants has not come into significant differences in the outcome of the patients. For example there's the polyurethane implants by Polytech (german brand) or the microtexturized implants by Motiva . I have used both Motiva and Polytech and are excellent implants. I have also used almost all the brands mentioned (Allergan, Mentor, Sientra) and other brands (Eurosilicone, Silimed, Nagor, etc), the choice is the surgeons preference regarding his experience. Still I don't have any trouble in using FDA implants if patients request them.

All of the American implant manufacturers
make high quality products.  They are all
FDA approved and are good devices.  So,
really it is a matter of personal preference as to which brand the surgeon (and
his patient) prefers.