Step 1: Reason for high oxidation state stability.
High oxidation states are stabilized by highly electronegative ligands because they can withdraw electron density from metal strongly.
Step 2: Compare halogens.
Fluorine is the most electronegative and smallest halogen.
So it forms strongest M–F bonds and stabilizes metals in very high oxidation states.
Step 3: Conclusion.
Thus transition metals show highest oxidation states in fluorides.
Final Answer:
\[
\boxed{\text{fluorides}}
\]