Conceivably, you could use either one.
If you say: These donuts are works of art.
that means each individual donut is a work of art.
However, if you say: The donuts are a work of art.
that means the donuts (collectively, perhaps in box of 12) look like a work of art.
The key question: Is this one work of art? Or twelve works of art?
By the way, the phrase work(s) of art would generally refer to how the donuts look, not to how satisfying they are. If you want to talk about taste and satisfaction, you could use the similar term masterpiece: This donut is a masterpiece!. The word is defined at Dictionary.com as:
1. a person's greatest piece of work, as in an art.
2. anything done with masterly skill: a masterpiece of improvisation.
3. a consummate example of skill or excellence of any kind:
The chef's cake was a masterpiece.
whereas work of art is defined as:
work of art (n.)
1. a piece of creative work in the arts, especially a painting or sculpture.
2. a product that gives aesthetic pleasure and that can be judged separately from any utilitarian considerations.
That part about "aesthetic pleasure" is a key part of the definition, I think.