Gura Gura no Mi, Magu Magu no Mi and Hie Hie no Mi are all incredibly dangerous and powerful fruits. They go beyond combat, these are fruits with the capability of altering the very climate of the world itself, as Magu Magu no Mi and Hie Hie no Mi have already proven. The Gura Gura no Mi allowed Whitebeard to tilt an entire island and the sea around it, causing incomparable destruction as well as natural disasters (tsunami's, earthquakes). I could easily see it do even more devastating damage if used cleverly: it can tilt and rise up continental plating with earthquakes, which has the potential of changing sea currents, which changes the very climate of the world. The power of this fruits is really mindblowing. I don't see the Yami Yami no Mi doing this very soon.
Of course, that depends on what one considers "strongest". In terms of sheer power, it's hard to beat the three I mentioned. But the Pika Pika no Mi, Goro Goro no Mi and Yami Yami no Mi are also pretty broken to the point of enabling the "god mode" cheat on whoever eats them. But on the other hand, Marco's Phoenix fruit, even though it doesn't cause mass destruction and climate changes, makes Marco pretty much invincible since he can even shrug off Haki attacks (unlike logia's, if the haki is strong enough, that is). That's a very strong fruit, too, though in an entirely different way than the other ones I mentioned.
