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.

dear Patient: In my practice I use Natrelle Inspira by Allergan.  I also use their fillers Juvederm, Voluma and their Botox (all Allergan products).  Their Inspira cohesive gels in the high profile range provide my patients with a very natural, projected results. Hope this helps!

Thank you for your question. I don't have any experience with Nagor or Arion, however I have a lot of experience with the big 3 in the USA which are Mentor, Allergan and Sientra. With the three popular US brands I don't honestly believe there is a big difference between them. It is really comparing apples to apples. Most surgeons will use one brand as their primary brand so they order enough they get better pricing. Buying 1/3, 1/3 and 1/3 really doesn't get you the buying power that 9/10 does. In addition the customer service and how easy the companies are to work with goes a long way. I don't think one brand is "better" than another, it comes down to the surgeon's comfort, preference and pricing. You should have a consultation to determine what your surgeon thinks is best for you. Best of luck.

Of the three that you mention, Mentor is the most commonly used in the United States.  The big three often used in the US are Mentor, Allergan, and Sientra.  The implant selection should be based on a combination of your desired size, soft tissue dynamics, breast base diameter and skin elasticity.  I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with.   Finally, make sure your surgeon is a Double-Board Certified Plastic Surgeon certified by the American Board of Plastic Surgery, a member of the American Society of Plastic Surgery (ASPS) and the American Society of Aesthetic Plastic Surgery (ASAPS).Best,Dr. DesaiBeverly Hills Institute for Aesthetic Plastic SurgeryHarvard Educated, Beverly Hills & Miami Beach Trained, Double-Board Certified Plastic Surgeon