I read somewhere that DFS is a special case of Best first search algorithm if f(n)=-depth(n). please justify this i am not getting it.:/
Consider a node $u$ such that $depth(u)=l$.
In Best first Search when you evaluate $u$ with the evaluation function you defined, you have $f(u)=−l$.
queue ╔═══╤═══╤═════╤═══╗ ║ r │ a │ ... │ u ║ ╚═══╧═══╧═════╧═══╝
When you expand $u$ and you evaluate the first child of $u$ (suppose it is Node $v$) you have $f(v)=−(l+1)<−l=f(u)$ so now you expand $v$ (that means that you put $v$ in the front of the queue) and you continue.
queue ╔═══╤═══╤═════╤═══╤═══╗ ║ r │ a │ ... │ u │ v ║ ╚═══╧═══╧═════╧═══╧═══╝
This corresponds to what you do when you use Depth First Search.