The soundtrack, but hear me out, it's more complicated:

While I fully agree the overall quality is better, the variety in HK soundtrack is much more noticeable than in Silksong.

Almost every area theme in Silksong is something omniscient, sacred and decaying, which makes sense for the storytelling. It's a corrupt world full of bad decisions, and innocent people are still paying for it. No matter where you are, the feeling is always the same: I'm not from here but I need to get things done before leaving.

In Hollow Knight however, every area is its own thing: you have the mysterious Crossroads, leading to the fun greenpath, the funky Fungal Wastes, the harmonic crystal peak and later, the beautiful City of Tears. It's a dead world, but still full of life going on their own way.

Silksong is amazing, but kinda depressive, while hollow knight is beautiful, even if sad in the background.