Whole Cake Island was at the very least, a self contained mess. Everything I disliked about WCI only affected my enjoyment of WCI itself.
Wano however is such an absolute, disgusting dumpster fire that it retroactively shits on the whole series and it affects my enjoyment of One Piece as a whole.
The Whitebeard Pirates were reduced to being absolute bitches, all of the Yonko and legendary pirates (bar Roger) of the previous age were reduced to lackeys of some generic big bad with a name that looks like it went through a randomized word generator, the World Government looked absolutely fucking stupid over the Nika Fruit nonsense and of course, the retroactive middle finger that is the Nika Fruit itself.
So based on this, WCI is the better arc since it didn't derail the whole damn story.
Tis best not to tempt both fate
and Oda.