Can anyone recommend a good knife sharpener?

I’m looking at Christmas gifts for my hunting buddy who has it all in the way of gear, but since I can’t justify buying anything other than gear, I think I’ll get him I knife sharpener. I mean, you can never have too many, right? But of course there are a multitude of sharpeners in varying styles, honing materials and degrees of compactness. Can anyone recommend a good one for an outdoorsman?