I've used a lot of Rapala knives- both black rubber & wooden handle. Preference is for 6" on panfish and smaller catfish, a 4" fillet knife for small panfish/perch that you'd like to fillet. 7.5" knife is preferred on trout, pike, and walleye, but I'd also give the nod to a stiffer blade/straight boning knife for those, as you need to break through the ribs. 9" for salmon, for sure, and for skinning pike & large catfish fillets.
I have some of the Kershaw fillet knives in 7.5" right now, and I feel like the steel is just a little too soft- the edge is rolling over more than chipping- it's not ideal for rib-breaking on walleye, but it works well enough. They do well enough for boning out ungulates, but still prefer a 6" curved semi-flex Victorinox for that.
19 degree blade angle is a good one for fillet knives- this isn't a pocket knife/field dress/gutless method knife, so you can stand a low angle since it doesn't have the same demand on it.
Best knife was a custom one with deer antler from a ferrier in SE Missouri- I wish I knew what steel he used, because that knife will take & hold an edge like nothing I've ever seen. It held up at much lower angles than 19 (never measured it, but it was noticeably lower than a Rapala).
Buck folding fillet knives aren't bad in 5-6", I don't like the handles on Forschner/Dexter (even though it's industry standard), and I feel like Rapala gives the best value, as long as you keep up with honing the blade.
