My favorite Final Fantasy: VII summon is Hades. Not because he looks kinda funky, but more the point it's PERFECT for this little combo -
Weapon Materia - Added Effect + Hades. Cover 100%. Counter-Attack. Counter + Mug (optional).
Armour Materia - HP+. HP+.
Your character will now block every physical attack and counter-attack with a large range of status-effects. Keep the Materia to a minimum to make sure your attack and vitality are at their highest for maximum effect.