I recommend Stubai. Read the review on the Diefenbacher website, as I own a set I entirely agree with it. They hold the edge well, when I am slamming (!) the blade into doug fir using a beech mallet, they do not take niks in the cutting edge.
I have sharpened the Stubais side by side with a Bahco/Sandvik (lots on eBay) on Norton yellow 220 A/O and the Stuabi leaves a light grey dust, a fast and easy to attain edge that is glass_smooth_razor_sharp. Perfect quality steel.
The Sandvik (Sweden) leaves it very dark and "gummy" (so to speak) and a harder to attain, not_as_good edge. These equalize with the cheap chrome vanadium chisels from woodworker's supply, a set of which I have (blue plastic handles, super cheap).
I have new Buck bros. chisels (hickory handles with leather tops) that are a better steel that the Sandviks but they are RC 59 and did take some niks, not hard enough. When sharpening, they leave a non gummy color that is merely darker than the Stubais, but a powder and not as dark as the Sandvik.
Other folks in here recommend Two Cherries (a TON!), lots of hard work to flatten the backs from too much machine polishing.
Stuabi are cheaper and just as worth it:
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Austria, very flat backs. Don't let the low price fool you.