Adele. She can write and sing. The other two only mastered one of the two.

Adele > Taylor >>> Ariana

I'm not a fan of Taylor, but death at people discrediting her or at least ranking her below Ariana.

Ariana is nothing but an unrefined vocalist. Taylor might not have mastered any department of her skill set, but she's at least decent in all aspects. I find Ariana to be severely overrated in the one thing she is known for.