I know you want the best, but I have found that those cheap Rapala wooden-handled fillet knives (around $13) has lasted longer than a lot of other fillet knives I've owned. I've got one that I've had for more than 5 years...
My wooden handle Rapala knife is at least 3 decades old and has outlasted many expensive knives.
My son has a Buck folding fillet knife, which is about the same age. He uses it for his hunting fillet the meat to back pack it out, as his duck breast filleting and other critters.
I have a 8 inch H&K fillet knife which my wife bought. It matches her knife set. It has the non slip composite handle that never soaks up blood, water or whatever. I use it for the bigger fish. Often Costco and the fish markets up here will have Halibut and large chunks of Salmon at a big savings in you can fillet the chunks. Between the two knifes, filleting is no problem.
Two cautions with fillet knives:
1. They are not meat/bone saws or axes. Never use them in that manner.
2. If they are as sharp as they should be. Be very careful using them or you might fillet
a finger or part of your hand.
When you own a fillet knife or any knife, sharpening them for most us is a challenge and can be expensive re some of sharpeners. Most of them never work for me.
What works best and the easiest is to use the bottom of any ceramic coffee mug. It is fast and a lot safer than most knife sharpeners. After I get the edge the first time with any knife, which takes a couple of minutes, the coffee mug can rehone my knives in about a minute per side. I try to use the coffee mug after each use.
This system works with every knife with the exception of the serrated blades.
The youtube video shows how easy, simple and effective the bottom of a coffee mug is to sharpen a knife. I find this to be a lot safer, easier and faster than any commericial knife sharper.http://www.youtube.com/watch?v=1TqXh2E3Auk