which

[Ju]

1. Which of this group of students are the most outstanding one?

2. Which of these dresses fit you most?

************************************************** *********

Which can be used to refer both objects and human. Am I right?

Thank you.

[emsr2d2]

1. Which of this group of students are is the most outstanding one? ("Is" should agree with "one").
Which of this group of students are the most outstanding ones?
Which of this group of students is/are the most outstanding?

However, I would prefer "Who of this group ..." in every case, even though "which" is not totally incorrect.

2. Which of these dresses fit you most best? (Suggests a plural response.)
Which of these dresses fits you best? (Suggests a singular response.)
Note that we don't use "most" with "to fit" in this way.

************************************************** *********

Which can be used to refer both objects and human. Am I right?

Thank you.

You're right that it can be used for animate and inanimate objects. However, your sentences have some problems. Please see my comments/amendments above.

For your first example, I would probably use "Who is the most outstanding student in this group?" or "Who are the most outstanding students in this group?"
"Who" replaces "Which student".