Many knife manufacturers provide the hardness value for the knife blade in the specifications - like 53HRC or 57 HRC.
What's the optimal hardness for kitchen knives? Do I always prefer the ones with higher hardness all else being equal?
Unfortunately, even hardness is a trade-off, all other things (like maintenance, appearance, balance, thickness, and so on) being equal:
I think the manufacturer's indicate the type of alloy they use to help indicate the quality of their product, as opposed to more generic knives. Things aren't always equal, though--in choosing a knife, I would not consider this fact one of the more important ones compared to how it feels in your hand, the thickness of the blade, how much maintenance it requires (for example, so-called carbon steel knives need more loving care).
There is no single optimal hardness for a chef's knife. It depends on:
Generally high performance chef's knives will run in the 57 to 63 range for hardness. It's a MYTH that a 63HRC knife will perform better than a 57HRC knife. The harder knife may chip more easily, lose its edge more easily, etc. depending on the factors above.
Some other users said hard steels are brittle and chip easily but this not always true. The same steel if hardened above its possibilities gets more brittle but if you compare different steels it's a different story. Some steels are very hard and if well manufactured they will last long without chipping.
High carbon steels (such as M2) are hard (you can find them with HRC 62) but they get rusty easily.
Powdered crucible steels are very good and usually more rust resistant. Among the best steels you can find Cowry X, ZDP-189 (HRC 66) but they are really expensive. A little bit cheaper and softer are SG2, HAP-40 and Hitachi blue and white. Other good steels are SV30, SV35 and SV90, or cheaper alternatives like VG10.
Hardness is not the only parameter here, some steels are hard but don't achieve a good edge because they are made with coarse particles.
The way the knife is manufactured also affects its specifications. For example Damscus knives are built in a complex and long forging process that creates a multilayered steel, very resistant to chipping and very hard. Laser and cryogenic treatments are also common.
Special coatings (with chemical vapor deposition) can be added to further increase the hardness, it's very common to use tungsten carbide.
If you want to cut medium or soft materials the harder the steel the better, it all depends on your budget. If you want to cut very hard materials, and you do it by hitting it, then you would need to be careful with brittle steels. In the kitchen you only have this problem when breaking bones.
If you select knives by hardness, that only makes sense if the knife is also made of a steel that can support that hardness well.
Note that the hardness of the steel will not have any bearing on its stiffness, as counterintuitive as it seems. But: The softer a knife, the easier it will permanently bend, or flatten when impacting a board. The harder, the more force will only cause it to elastically deform then spring back. Harder also means a force working on it will not be relieved by bending it away. What cannot bend, and is at the end of its elasticity, will break. Force needed to actually break is dependent on the steel type, and tends to be very little if the steel is hardened above what is recommendable for it.
So a very soft steel will just bend away when overloaded, requiring you to bend it back afterwards. A very hard steel will be tensioned more and more, until it snaps. Which can result in small pieces of your edge being lost, giving you a blunt knife, or the actual blade snapping.
What I'm giving here are experience values from having handled quite a couple of knives keeping an eye on what holds an edge well and what doesn't:
For those used for straight push cutting/chopping/slicing with controlled force, like a Santoku, Nakiri, Gyuto, 60-64HRC. Suitable steels: VG10, non-stainless (carbon), PM steels.
For a chef knife used in a rock-chopping/walking/mincing manner, 57-60. Harder, more brittle steels tend to microchip from being loaded sideways.
Anything used to chop with momentum, or to lever with, or to cut very cold or hot foods: softer, 54-56HRC.
Anything used for controlled slicing on softer ingredients, without impacting the cutting board, or for VERY careful board work (eg Sashimi knives or Usubas): 62-69 HRC, but mind that very hard edges usually need a more obtuse edge angle on a board than something in the sub-65HRC region. Carbon and PM steels.
For a thin knife blade you want a 16 carbon. A good spring steel. For a thicker knife a 18 carbon. A stiff spring steel. Holds a edge a little longer. Over that steel becomes brittle. Will break easy. 14 carbon is a weak spring steel. Dulls fast. These are blacksmith figures on hard steel. 0 pig iron. 24 carbon dimond steel very hard & brittle.