worst-case OWF from weak OWF
In order to answer this question, you first really need to define what a worst-case OWF is. In particular, is it possible to efficiently sample a hard instance? I assume not, because this would be very hard to formalize and I'm not sure it makes sense. If not, then there is an easy answer:


If there exists a weak OWF then $P \neq NP$
If $P \neq NP$ then let $L\in NP\setminus P$ and let $R_L$ be its associated polynomial-time verifiable relation (i.e., $(x,y)\in R_L$ if and only if $x\in L$).
By the definition of a "worst-case" OWF, take a machine that samples $(x,y)\in R_L$ and outputs $x$. 
The function would be defined to be $f(y)=x$.