This is what I carry in the field:
Eze-Lap Folding Double sided diamond fine/med
That said I never use it.
Mainly because of the Havalon knives I now carry.
But I'm a believer that everyone should carry a sharpener for long term survival reasons. Particularly for fixed blade & hatchet needs, not that we have to carry it all the time, but it should be in backcountry kit.
In my UL kill kit, which is on me ALL the time, I carry a mico 1"x3" double sided finger nail file. Weighs nothing and will do the job in a pinch.
Hunt'nFish
For kitchen knives, hunting knives, etc., I use the EdgePro system and love it (21 deg edges mostly). In the field, it's an easy touch up w/ his ceramic hone. For single blade broadheads, 3-blade, and 4-blade like slick tricks, I use a mill file first and finish w/ a strop in most cases (st's sometimes get a diamond hone though). The edges are durable for me when doing the heads this way.
+1 for Fallkniven DC3 or DC4. I got the larger of the two. Lightweight, works like a champ. Grit is fairly high (for hunting)- I think about 600-800, so if you let the knife get too dull or nick the blade, be ready to sharpen for a loooong time. If you keep on top of sharpening, these are great for touching up an edge.