She is . . . . . . . . most famous actress I know.

The correct answer is: no article.

The word "most" is a superlative, which means that it is used to compare things that are in a group. In this case, the group is "actresses". The speaker is saying that the person they are talking about is the most famous actress in the group of actresses that they know.

If we use an article, it would imply that there is only one actress in the group. For example, if we said "She is the most famous actress I know", it would mean that the speaker only knows one actress. However, the speaker could know many actresses, and they are saying that the person they are talking about is the most famous of all of them.

Therefore, the correct answer is "no article".