Raw or Cooked?
This is a bit of a personal preference. Both are perfectly safe (as long as the cooked eggs aren't hot!). Raw, however, is more nutritious. I find that most ferrets seem more keen to try raw eggs. It's also easier to dab on their mouth to convince them to try some.
Whole, yolk only, or whites?
This is where much of the confusion about feeding eggs arises. What parts of the egg are safe for ferrets. One of the biggest points I read about feeding raw eggs is that the whites should never be fed. This is correct. Egg whites contain avidin, which binds to biotin and over time can cause a biotin deficiency. Sounds scary right? Well the good news is that egg yolks contain a ton of biotin, which basically counteracts the effects (LOL I typed "egg") of the avidin.
So are whole eggs safe to feed? Yes. I do suggest that you whisk up the egg first, so the yolk and white are mixed, that way it is eaten in equal amounts.
Yolks-only are also a perfectly acceptable option, and if you have a single ferret, may be a more appropriate serving size.
Eggs are pretty rich, and can cause really funky poops when fed in excess amounts. (ask me how I know)
It's usually recommended to limit them to treats 1-2 times a week.
They can be given more often during shedding season to help hair pass through, but beware the funky poops!