Magnify Materia is by far the best materia available in the game. It allows you to multiply the effect of a materia that is attached to Magnify. For example, you can use Magnify Materia + Ice Materia to use Blizzaga on all enemies (with minor reduction of damage as a penalty). How do I get this powerful item? You can find out about it in the following chapter of our guide to FF7 Remake.
Magnify Materia can be obtained in Chapter 9 - The Town That Never Sleeps. Near the third puzzle (with the robot hand), you can flip Aerith to the Materia that lies on the ground near the entrance to the tunnel. The item must be acquired because it is the only Magnify Materia available in the game. Unfortunately, you can't get another one, even on Hard Mode.
Magnify Materia is best combined with Healing Materia to heal the entire team with one spell. This is especially useful on Hard Mode - on this difficulty level you can't use any items. The item is very useful in later stages of the game. Therefore, remember to make full use of it.