Fire + All + Mp Absorb
This one will strike all enemies with Fire 3 plus you get 10% of the amount of health you damages. For example, if the total damage was 1000, you get 10 MP.
Fire + Quadra Magic + Added Cut
Ouch, this one hurts! The character who fires this spell will hurt the enemy four times and in the end even attack one more time with the weapon.
Fire + Elemental Materia on the Weapon + Death Blow
When you attack the enemy with your weapon using Death Blow he will get fire damage doubled. Just imagine if he is weak to fire attacks such as an ice creature!
Fire + Hp Absorb + Steal As Well
Now only you grab 10% of the damage inflicted in HP but you snatch the enemy item too! This is specially good for enemies with rare items like the Circlet in the Icicle Area or the Champion Belt Ring in the Battle Arena.
Finally
Counter + Mime
Simply one of the best materia combinations you can ever make. Just using this one you can beat almost any single enemy automatically.
Beginners read this carefully, kill two characters and leave only Cloud. Now equip him with a good magical attack such as Magic Breath Enemy Skill or Comet 2 Magic and link that materia with an MP absorb and HP Absorb.
Now every time Cloud is attacked he will mime his own attack and get all his HP back and kills the enemy all by automatically!