Adhesives from reputable companies are cosmetic quality and any product classed as a cosmetic should be safe for the skin (with relevant warnings of course). True medical quality is very expensive and must be sterile etc etc but good nail adhesives are as close to that as possible.
As to the best one to use, it depends on the job it needs to do and personal preference. Very thin adhesives are usually faster and great for a smooth nail plate that has a perfect contact with the tip. Thicker adhesives are good to fill up and cushion the irregularities on the nail plate.
Some people prefer to use a thicker adhesive as there is less chance of air bubbles. So a thicker adhesive, like Gel Bond, would be perfect for replacing a lost nail (as long as there is no allergy to the cyanoacrylate)