Off the top of my head, I can think of a couple DFs with incredible potentials that were wasted on mediocre/scrub-leveled characters.
Mr. 5's Bomb-Bomb fruit, just to name an obvious one.
Some can argue that if Buggy's fruit were in the hands of a swordsman, it would be extremely overpowered.
The Noro-Noro and the Horo-Horo are hard ones to argue, as the Noro-Noro beam itself is actually rather slow (and dodgeable), while the Horo-Horo… I guess it's far too overpowered assuming that you're not fighting Usopp (even when Perona was using it)
Special Add On Question: Choose a fruit, and say how you would use it if you were the user (use of weapons, etc etc)