For me it's Astra SP - aka The Devil's Blade. That thin sliver of hatred cut me to ribbons and seemingly caused some kind of reaction on my uncut skin that took nearly a week to go away. I think I'd have got a better shave using the lid from an old open can of beans that had somehow dodged the dustbin.
My technique has improved a lot since then but I still can't bring myself to try Astra SP again. I haven't even summoned the courage to try a pack of Astra SS that I have lying around.
Most of the time nicks and cuts are due to improper shaving technique rather than bad blades. I do not like Astra SP blades as they are not sharp enough for my coarse beard, but I have never been cut by one. For many, presumable those who do not have tough beards, like them.
Astra SS blades are very different than Astra SP. The SS blades are much sharper. I put them at the sharper end of the mid-sharp category. They are sharp, smooth and consistent. That makes them one of my favorite blades. I can use them in any razor from very mild to very aggressive.