This is a tough question to answer, since devil fruits would have to be judged on the following criteria: the fighting ability and creativity of the user, the potential and limitations of the fruit itself, the synergy between fruit and user, the user's experience with the fruit being chief among them, and the situational circumstances of the user. Since as far as I know, the fruits don't tell their users their secrets, and it's hard to gauge the end potential of a fruit since we have generally seen only one user per fruit (with two exceptions so far; three if Miss Valentine died and Machvise ate the reincarnated Kilo Kilo Fruit as I assume).
For instance, Brook's devil fruit was considered to be useless save for having revived him after he died, but he learned to astrally project himself and channel the cold of the underworld, so I would call that good use of a devil fruit.
Sugar's devil fruit has a very specific usage, but the fact that it's super overpowered and she used it to pretty much take control of an entire country on Doflamingo's behalf means it was used pretty damn well.
On the other hand, any logia user that thinks himself invincible isn't likely to draw out the fruit's full potential, and the likes of Caribou and Gasparde could be said to use their powerful fruits rather poorly by New World standards.
Kalifa and Kaku were forced to use and improvise with their newly acquired devil fruit powers, but they showed creativity and willingness to experiment even in a life or death battle, that is pretty good use of a devil fruit, all circumstances considered.
Rocinante may not have used his devil fruit powers a lot, due to wanting to keep them secret from Doflamingo, and from what we have seen the fruit's powers may be fairly limited in scope, but he still managed to get Law out of the bird cage alive.
Kanjuro and his devil fruit, which lets him bring his drawings to life, suffer from bad synergy since his incredibly bad drawing skill likely keeps him from bringing out the fruit's maximum potential. Could that be considered a bad usage of the fruit? Not by itself, I would say.
As for the best usage of devil fruits seen up till now, I would have to say Luffy, Doflamingo and Law. Luffy as the main character gets to show off a lot of abilities, and there is no doubt he can do some crazy shit with his devil fruit. It is fair to say he has plumbed depths that few fruit users would, as I have seen very few devil fruit abilities that are as physically and mentally intensive as Luffy's Gears. Doflamingo, for obvious reasons (Awakening) and Law because he uses his intensely powerful fruit to create a massive variety of effects.
As for worst uses? Well, Senor Pink springs to mind. I really like the guy, but his hard-boiled attitude makes him use his fruit rather poorly, considering he could likely just draw Franky under and make him suffocate. Which is fine. This manga wouldn't be quite as good if everyone used their fruits to perfect effect.
