Normal Humans > Devil Fruit Users ! All Devil Fruit Users have kryptonies that can turnover any battle, (ex Luffy defeated Crocodile using water.)
As for the most powerful Devil Fruit ? I'd have to say Logia, hands down.
Logias can:
1) produce limitless element/substance out of nowhere
2) become invincible to "physical" attacks
3) modify their bodies
4) become unaffected by element/substance
5) control element/substance
6) take advantage of element/substance properties (ex Kizaru can move at speed of light)
The other Devil Fruits just can't hold a candle to Logias.
... I might rethink the whole "Normal Humans > Devil Fruit Users" thing.
Well that's just on paper, of course the user behind the Devil Fruit matters too and blah blah blah.
Logia type, since they are the least vulnerable but it really doesn't matter if they can't fight or fight someone much better, or come across a swimming pool
The Logia's are the strongest. Rivalled by the Mythical Zoan's for example Marco's because he's a phoenix and can instantly regenerate. Everything else can become super strong like Luffy's but for a base fruit, then the likes of Kizaru have the strongest abilities.
I mean, how do you beat Light? Other than with like Blackbeard's Darkness fruit. But then does light beat dark, or vice versa?
Hey anyone remember what Crocodile said long ago? All devil fruits can be really powerful as Logia, it all depends on the user, training and discovering new uses and abilities is what would make any user very powerful no matter what fruit they have eaten, just like Luffy has done with the Gum Gum.
Also would say that normal strength could be the best because Haki, like Shanks for example he is a Yonkou and hasn't shown signs of a Devil fruit's ability.