If cyclingcols is correct it's 5kms at 14.5% vs the Zoncolan's 5kms at 15.5%, so Zoncolan is harder. So this time I can actually say that I've climbed worse.
Zoncolan is the worst climb that I've done because the hard part is so long, in an actual race the old road to Oberbozen with 1km at 21% and 3kms at 17.4% is probably worse, but if you climb it on a training ride you can take it easy on the 8-10% sections afterwards. The hardest part is the hardest thing I've ever ridden, but with it being at the start of the climb it makes the overall climb less hard than the Zoncolan on a training ride (would probably make for better racing and produce bigger gaps because everything explodes right after you hit the climb). That climb is also a horror in the summer, the super steep part is exposed to the sun (surrounded by Vineyards) and Bozen is often one of the hottest cities in Italy, you hit that climb with well over 35° in the afternoon. For reference it's this climb minus the final 600m:
https://www.cronoescalada.com/puertos/viewProfile/3411
You have 300m on a private road, that's why the sign calls it a dead end road.