Sailing 'westabout' around which cape is considered the hardest passage?

Cape Horn, and the direction is the whole difficulty. Going east, the wind and the current are behind you; going west you are pushing into the prevailing westerlies and the Antarctic Circumpolar Current, in the one latitude band where nothing interrupts either.

That band is the reason. Below about 40 degrees south there is almost no land, so wind and swell circle the planet uninterrupted and build without limit - the roaring forties, the furious fifties, and waves that regularly run over ten metres. At the Horn the seabed shoals and the Drake Passage squeezes the whole flow between South America and Antarctica, which steepens everything.

Square-riggers carrying grain and nitrate did it westabout for decades and sometimes failed: one ship beat against it for over ninety days and gave up, turning around to reach Chile by going the other way round the world. The Panama Canal ended the trade in 1914. The passage is now sport, and a solo non-stop westabout rounding remains one of the harder things anyone does in a boat.