Your best employee. Would you miss them if they were gone?
The answer should be yes and if it is you should take good care of them. If the answer is no, you’ve got a lot of recruiting to do. If they are your best and you wouldn’t miss them, then you probably wouldn’t miss any of the others either.
If you are in good shape, you would probably miss a good number of them if they were gone. The ones that you wouldn’t miss are your top candidates for replacement.