It's so relative question , and even everyone can give a lot of answers .
I can understand it in 2 ways :
first : Which is the thing that could be my worst friend and best enemy .?
secondo:which is the two things , that first one can be my best friend and second can be my best enemy ?
Well, i try to give answers for both of the meanings ( I cannot guess the real meaning :-) )
First : I'm My own worst enemy and best friend ( it's an extremist answer :-) )
Secondo:My worst friend is PLEASURE , and best enemy PAIN .
- My worst friend is the one