What is the best knife steel if I only want edge retention and toughness?

Easy answer….follow the carbon content of the steel and additives. Here a copy of an article I read.
400 Series
420 Steel : Common in lower-end knives, this steel has about .38% carbon. The low carbon content means that it’s very soft in comparison to most others, and it doesn’t hold an edge well. Blades made from this material need to be sharpened frequently and are more likely to chip. On the bright side, all 420 stainless steel is extremely rust-resistant. This makes it a great material for diving knives since their constant contact with salt water makes them more likely to rust.
425M Steel: This is a material similar to the 400 series that has .5% carbon. Not a great steel by any means, but it’s not a bad one either, and it’s reasonably tough for the price.
440 Steel : There are three different types of 440 steel: 440A, 440B, and 440C. The further along in the alphabet, the better it gets. The only problem is that oftentimes manufacturers simply stamp “440” on the tang of the blade without the letter grade, so knowing what you’re buying is tricky. As a rule of thumb, if it doesn’t explicitly say 440C, it’s most likely a lower-end version like 440A or 440B. Certain knife manufacturers have even gone so far as to rename 440C as other things in order to differentiate the quality of the product. 440A Steel : This low-cost stainless steel has a carbon content range of .65-.75%. It is the most rust resistant of 440 steel, and 440C is the least rust resistant of the three. However, taken as a whole, the 400 series is comprised of some of the most rust-resistant steel you can buy. 440B Steel : This is very similar to 440A, but has a higher carbon content range (.75-.95%), so it has better edge retention. 440C Steel : This steel has a carbon content range between .95-1.20%, and is generally considered a higher-end steel. It’s extremely common in knives because it provides a good mixture of hardness and corrosion resistance, but also isn’t terribly expensive.
AUS Series
The biggest improvement of the AUS series ( made in Japan) over the 400 Series is the addition of vanadium, which improves wear resistance and provides great toughness. This also reportedly makes the steel easier to sharpen.
AUS-6 Steel : AUS-6 Steel has .65% carbon and is generally considered a low-quality steel. It’s comparable to 420, but with better edge retention and less corrosion resistance.
AUS-8 Steel : This fairly popular steel has .75% carbon, which makes it fairly tough. It also has more vanadium than AUS-6, so it holds an edge better. Cold Steel likes to use this stuff a lot in its blades
AUS-10 Steel: Our final option in this series has 1.1% carbon and is roughly comparable to 440C. It has more vanadium and less chromium than 440C, which makes it slightly tougher in comparison, but is also a little less rust resistant.
ATS Series
ATS 34 Steel : This steel is very similar to 154 CM (listed below), and is generally considered one of the best steels you can buy. It has 1.05% carbon, and there are lots of high-end custom knives that use it because it’s just excellent metal. Benchmade and Spyderco seem to be fans of this steel, as you’ll see it a lot in their products.
ATS 55 Steel : This steel (carbon content of 1.00%) does not have the vanadium that is present in both ATS-34 and 154-CM (see below). This means that it doesn’t hold an edge as well, and has also been reported to be less rust resistant than ATS-34.
SXXV Series
This series is becoming quite popular because of its strength, ability to resist rust, and how well it holds an edge. These are difficult steels to sharpen though if you do need to give them an edge. All of these knives are very wear resistant. The 30, 60, and 90 you’ll see in this series stand for 3%, 6%, and 9% vanadium in the alloy, respectively.
S30V Steel : This steel was designed to be used for knives. This steel is very tough, and yet still has great wear resistance. For how tough the steel is, it actually has very good hardness, too, which is why many consider it to be one of the best choices for knife making. It has a carbon content of 1.45%.
S60V Steel : This stainless steel has high wear resistance. It has lots of vanadium and also a carbon content of 2.15%. It’s just a step above S30V but is relatively rare in knives because it’s more expensive.
S90V Steel : This steel has superior edge retention, but due to its ridiculous hardness, it can be almost impossible to sharpen. You’ll typically only find it in custom-made knives, and it has a carbon content of around 2.30%.
Others Noteworthy Steels
154 CM Steel : This is high-quality steel — arguably one of the best available for knives. It has a carbon content of 1.05%, it holds an edge well, and has pretty good toughness for how hard the steel is. It is tougher than 440C and is often compared to ATS 34 because the two are so similar. Some people prefer this steel to ATS 34, however, since this one is made by Crucible , an American company.
M390 Steel : M390 as 1.9% carbon, is very stain-resistant, and has excellent wear resistance. It has vanadium as an additive and consequently is a popular hard steel. This is also the type of steel used most often for surgical applications.
N680 Steel : Last but not least, N680 has .54% carbon. This is another very hard steel that is highly stain resistant, making it good for saltwater applications.
Article originally published September 12, 2016 by Drew Prindle. Last updated by Sam Slaughter.

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