Left to right:
Vintage Case Stainless
Murphy Knives (Ayerton MA) Carbon Steel... needs care, awesome blade for >$30
Kelgin (Orvis marked) Custom... ATS34 stainless, bamboo handle
Original USA Made Schrade Steelhead. Stainless... cleaned hundreds of fish with this one. My go to FK before the internet
Helle of Norway... Stainless, spooky sharp OOB but just a little short.
My favorite, and it is a meat eating animal is a Dozier custom
D2 steel, kydex sheath...
The custom maker known for his fillet knives is Phil Wilson.
A search for Seamount Knives will find him.
I don't typically carry my fillet knife while fishing, so straight carbon steels aren't a worry for me.
Besides, if you let the carbon steel patina (turn gray like an old hammer) it is less likely to form the bad orange rust.
You can hurry that patina along with a warm vinegar soak or smear it full of mustard and let it sit a while.
Go to one of the knife or blade related websites and search for 'forced patina'
The thing about titanium is that while it won't rust, it also won't hold a super edge or hold an edge for a long time.
Live everything else, there are trade offs.
If you are going to carry the knife (wet) all the time, and clean a fish only now and then, it'd probably be OK.
If you were cleaning a bunch of fish each trip out you'd be sharpening it a lot. And washing it a lot, so regular SS or even carbon would be a better bet.
After all that
How much do you want to spend?
If the internet never came along I'd still be happy with the Schrade Steelhead.
Or one of the Rapala's mentioned above (all my friends had them)
Having said that, I still may buy one of the white handled Dexter Russells... just cuz.
I have that same Schrade. I've been using it for a long time. Also have a DR that I like quite a bit. The best fillet knife is a sharp one. I really like te handle of that Kelgin and the looks of the Helle. I like to have three knives with me, or in the truck. A bait knife, a fillet knife and a heavier 'steaking' knife. oh yeah plus the benchmade pocket knife and the Leatherman :-)