Sissi asked me about knives for filleting fish. I thought about it, and here is a brief answer:
A stainless steel aji-kiri (aka ko-deba) and a sharpener, such as Global ko-deba with a blade length of 12 cm and Global Speed Sharpener.
As the name implies, an aji-kiri is a knife for filleting aji and other medium to small fish. The blade length ranges from 9 to 12 cm. An aji-kiri is also called a ko-deba (lit. small deba).
Images of aji-kiri
Images of ko-deba
A tradititional Japanese knife made of carbon/laminated steel and a Japanese whetstone are not for everyone.
I can think of other options like a mioroshi deba and the Global-Pro GP-5 Slicer, but they are much bigger and heavier.
A mioroshi deba (or bocho) is a variant to a deba that can be used to fillet fish and cut the fillet into sashimi.
I have one mioroshi deba, bought from Watanabe Blade. A picture can be found here.
The Global-Pro GP-5 Slicer, with a blade length of 210 mm, sounds like a truly versatile knife. I learned about this knife from this this blog (Japanese only), run by a professional Japanese chef. You can tell how versatile it is by looking at the photos.
Edited to add:
A santoku (double bevel) is another option, provided that it cuts well enough. You don't necessarily have to get a single bevel aji-kiri or ko-deba, especially if you don't have to fillet fish very often.
I learned that the SekiRyu ko-deba is a stainless steel knife with a blade length of 105 mm from a Japanese site. Scroll down to view a photo of the set of four knives. 2,890 yen! Really cheap.
A stainless steel knife is easy to maintain, should cut well enough (I know because I have a stainless steel petty knife from Watanabe Blade), but don't expect too much from it. It won't cut like a Global or traditional Japanese knife.
A #1000 and #3000 whetstone sounds good. (I sometimes wish I had a #3000 whetstone, but then again, I won't use a #3000 one very often.)
Don't forget that you have to get a whetstone fixer, too. Why not get a diamond whetstone
like I did?
I was like Sissi. Someone suggested that I should buy a good sashimi knife (> 10,000 yen), but I ended up buying a cheap one (< 3,000 yen). I don't want to use it any more. Then I bought a Global santoku, found it was good enough, but I didn't stop there. I wanted better ones, and bought a Shigefusa nakiri first and then a Shigefusa kitaeji petty knife. They are both very good, especially the latter. They need proper maintenance, but I love them!
In summary, buying the SekiRyu ko-deba (stainless steel) and the whetstone is not a bad idea, but as for the knife, you will simply get what you pay for.