She's the best (actress) (that) there is/exists.
The underlined portion modifies the implied noun "actress".
 She's the best (that) there is.
The underlined portion modifies substantive "the best".
Meaning: In the field of Acting, there is only one actress who is the best at what she does, and she is that actress. She is the best (actress that) there is (in her field of work).
Hope that helps.