So the strongest fruit category overall has some stinkers in it. But, what's the worst of the Logia fruit? IMO, I think it's Smoker's devil fruit.
Based on what? Who is going to get these DF powers?
If you have godly base stats that superior to L.admirals, then Darkness Logia is suitable for that person.
If you don't have superior base stats, then Darkness Logia is the worst. It doesn't give you an auto dodge ability of logia, plus gives extra pain when you get hit, only a true top tier can handle this DF power.
For example, for L.akainu, magma is better since he won't have the capacity to use Darkness Logia fully, while Blackbeard can.
For Blackbeard, Darkness is better, since he can use it to full potential better than L.akainu, Blackbeard can even steal the second DF power with it due to his insane base stats and durability, strength, and Haki level.
Without Darkness Logia in the equation, I say Swamp is probably the worst, he can be handled by Franky's trap if he is off-guard, that wouldn't happen with Smoke Logia or Sand Logia.
Also, Smoke logia gives speed boost, Swamp logia doesn't even give that. It has AoE via sucking the objects but I prefer Smoke's speed boost over that.