Mora's are always a real good bet. As are Dexter Russells, and I just picked up a Rapala Saltwater fillet knife at the grocery store that did a bang-up job on ocean Silvers earlier this week. I also tried one of the new Benchmade Salmon River fillet knives, and wasn't really impressed. I'm definitely more of a rigid blade person, and the BM felt flimsy, and didn't hold it's edge as well as the Rapala. (AUS-8 vs. Sandvik 12c27)
I tend to recall that Victorinox and Forschner make some decent ones too.
I once bought my father a Microtech Black Marlin fillet knife. That one I utterly loathe. I'm the one that always gets to resharpen it. (you know family and all) Once that S-30V has been through hundreds of pounds of salmon and halibut, it's a real bearcat to restore.
Or you could go totally traditional, like my mother did. She used a souvenier shop ulu and cleaned about four fish to our every one. she had the only Ulu out there- the rest of us had traditional fillet knives)
The Moras are easy to maintain and sharpen, and they're pretty darn expendable, but work very well.
Whatever you spend, I just wouldn't break the bank on it. I've reached the conclusion that expendability is a prerequisite for a good fishing knife.
Sorry for rambling. just had a real good fishing trip, and still a bit enthusiastic...
