Which filleting knife should I buy?

I like the Fibrox-handled Victorinox stuff (so does Cooks Illustrated), and think it's a great value, but it sounds like you want something a little fancier. Custom knives are definitely a thing, but there's a point of diminishing returns where you're paying more for exclusivity and artisan-ness than for utility or build quality. Think of high-end custom bicycles, or watches, or audiophile stuff, or bespoke clothing. It might be worth talking more to your dad about what he doesn't like about his current fillet knife--if it's something like the size or the handle material, that will provide good guidelines about what to buy.

Go to a high end culinary supply shop where you can handle the knives beforehand. I'm a fan of Henckels knives, but in my experience this stuff all depends on the individual and how the knife feels in your hand. Assuming you and your dad are of a similar build, you might be able to figure out the best knife to get if you go to a shop and try out a bunch of them. The salesperson should be able to show you the proper way to hold a fillet knift (then again, is your dad holding his fillet knife correctly?) Can you give your dad a card that says, "let's go knife shopping!" or the like, and then take him to such a shop to try out all the fillet knives? And then buy him whichever one he prefers. US chain shops that offer this sort of service are Williams-Sonoma and Sur La Table. Not sure if the UK has those, or if there is a better version where you are. Obviously, finding a culinary shop that is locally owned would be preferable, especially if run by someone who really knows their stuff. You're looking for a shop that has a knife section with a space for testing/handling the knives, and ideally a staff member working back there who specializes in selling knives (as opposed to the kid who just got thrown into "knife section" duty but knows fuck all about kitchen equipment).

You want forged knives rather than stamped. You want knives with a "full" tang (which is to say, knives in which the single piece of steel extends all the way through the handle). Here in the U.S. the decently upmarket-good-but-not-amazing-or-bank-breaking brands are Henckels and Wusthof, both German brands. I've had my Wusthof Classics for seven years now and you will have to pry them from my cold, dead hands. What box says is about diminishing returns is spot on. You can spend almost any amount of money on things like this if you want to.

Also, the Japanese vs. German thing, in this particular case, is probably not really an issue for your case. Since a fillet knife is a specific thing. The main difference between Japanese and German, in my understanding, is that the knife shapes and styles are different. For example the Santoku knife vs. the classic European Chef's knife. If your dad is already using a classic European-style fillet knife, you probably don't want to go crazy and get a Japanese knife, without careful examination that it is EXACTLY what he means when he says "fillet knife". If Japanese knife-makers even do fillet knives, of course. So that question is probably already somewhat answered for you. That said, Shun knives are pretty...

Is he filleting fish or beef? My parents have had a lake cabin since I was 9. My dad catches a lot of fish, especially now, since he's retired. If your dad is filleting fish, consider: 1. In Minnesota (Land of 10,000 Lakes, mind you) the http://www.leechlakeknife.com/Leech_Lake_Knife/Home.html is sort of the gold standard of full-tang fillet knives. 2. However, if you're mowing through a lot of fish, an http://www.cabelas.com/product/Hunting/Knives-Tools/Fillet-Knives-Fish-Processing%7C/pc/104791680/c/104740380/sc/104320980/Mister-Twister-Electric-Fillet-Knife-Combo/745062.uts?destination=%2Fcatalog%2Fbrowse%2Fhunting-knives-tools-fillet-knives-fish-processing%2F_%2FN-1100163&WTz_l=SEO%3Bcat104320980 may be what he wants. I know, not as sexy, and it makes noise, and it's not as cool as all of the artisan-craftsman-etc. knives, but if you want to cut a lot of fillets without being showy about it, it's worth considering.

My father is a butcher and echos http://ask.metafilter.com/217575/Which-filleting-knife-should-I-buy#3141493, and america's test kitchen's sentiments. For quality vs cost the fibrox handled Victorinox really can't be beat. Back when I worked at the butcher shop I used the http://www.amazon.com/Victorinox-Cutlery-6-Inch-Semi-Stiff-Boning/product-reviews/B0000CF94L/ref=dp_top_cm_cr_acr_txt?ie=UTF8&showViewpoints=1 for most everything and had no issues. The http://www.amazon.com/exec/obidos/ASIN/B0000CF8XV/metafilter-20/ref=nosim/ has worked just as well for me, although it doesn't get near as much use.

http://www.marttiini.fi/epages/MarttiiniShop.sf/en_GB/?ViewObjectID=10480 knives from Finland are quite decent and would not be a common choice.

Seconding Marttinii. All if mine are folded blades from Finland. If its for fish you want a really nice slim sproingy blade. The really expensive ones can be bent practically in half. Makes a big difference.

I really like knives and had a paring knife made by http://www.haslinger-culinary.com/available.html. It was indeed quite expensive. For ready-mades, I like Shun (see http://www.chefknivestogo.com/fillet-knives.html for a good deal on their fillet knife and also just a cool website in general - the difference between most MAC and Shun handles is evident here). http://www.surlatable.com/search/searchContainer.jsp?q=bob%20kramer&s=true Bob Kramer ready-mades are also nice. I know that I like heavy, small knives with a belly. Others like long, light ones or some variation thereof. How does your dad slice? Does he mostly use the end of the knife, or does he use the whole thing? As an aside, a book like http://www.chapters.indigo.ca/books/Knife-Skills-In-The-Kitchen-Charlie-Trotter/9780756633912-item.html?ikwid=knife+skills+in+the+kitchen&ikwsec=Books is a fun addition to a gift of a knife.