We've discussed the two types of Devil Fruits which are Paramecia and Zoan. So now we'll discuss the last type of Devil Fruit, the Logia.

Logia Devil Fruits are the rarest type of devil fruit and are considered to be the strongest type.  A Logia Devil Fruit can transform a person into a natural element and also control it.

Why are the Logia users considered to be the strongest?
1. Only a few weaknesses
2. Can only be touched using Haki

Who are the Logia users that we know and their skills?
1. Smoker - Moku Moku no Mi (Smoke-Smoke Fruit) allows the user to create, control, and transform into smoke
2. Crocodile - Suna Suna no Mi (Sand- Sand Fruit) allows the user to create, control, and transform into sand. From what we saw in Alabasta Arc, he can also dehydrate any non-metallic material (people, animals, the weather)
3. Portgas D. Ace and Sabo - Mera Mera no Mi (Flame Flame Fruit) allows the user to create, control, and transform into fire. After Ace's death, Sabo won it from Doflamingo's tournament.
4. Enel - Goro Goro no Mi (Rumble Rumble Fruit) allows the user to create, control and transform into lightning.
5. Marshall D. Teach - Yami Yami no Mi (Dark Dark Fruit) allows the user to create, control and transform into darkness. Using the darkness, he was able to pull White Beard's devil fruit capability which is the Gura Gura no Mi (Tremor Tremor Fruit)  that allows the user to create vibrations. 

Do you want to know more about devil fruits and their users, you can read them here.

https://onepiece.fandom.com/wiki/Logia