Question

Read the sentence.

Which discovery do you consider the most important?

What is the simple subject?

a. Which
b. discovery
c. important
d. you

Answer 1

Discovery, because that's whom the sentence is focusing on

Answer 2

The simple subject of this sentence is B. discovery.
Which is just an interrogative pronoun, it cannot be a subject. Important is an adjective, which cannot be a subject. You is an objective pronoun.