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).
It depends on a few different factors
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.