I have kind of a problem when it comes to fillet knives. I have a whole toolbox full of 'em. My favorites are marked below with a dot. From L to R, they are:
Dexter Russell V-Lo
Dexter Russell Sani-Safe
Knives of Alaska Coho
Knives of Alaska Steelheader
Rapala 7" (rubberized grip)
A very useful knife, if you're cleaning fish with tough scales (redfish for example) is the rightmost white-handled Dexter Russell. That one has a fully serrated blade.
If you want "RAT ideology" (which I assume to mean no BS), then get a Dexter Russell. There's a reason you see them with such frequency on charter boats. They're well-designed for the task of processing fish, and the high carbon stainless holds an edge. They're made in various sizes too. The V-Lo adds ergonomics to the mix. The V-Lo and the Rapala are probably my favorite two fillet knives pictured below.