For PURE defense which armor is best?

[Megalixir]

Not counting materia slots, those dont matter to me

Zeidrich, or Mystile?

[Captain Maxx Power]

Ziedrich by a long margin. While Mystile does have higher evasion there are quite a number of attacks which will ignore this. Ziedrich has the higher Def value and also cuts all damage in half.

[cloud21zidane16]

Mystile-Magic Defense 72 Defense 65
Ziedrich-Magic Defense 98 Defense 100

Go with the Ziedrich:)

[Megalixir]

my cloud setup is awesome.

In the back row,

Weapon:Apocalypse(Flash) (Pre-Empetive) (Speed Plus)
Armor:Zeidrich
Accessory:Fury Ring

so,
a super fast cloud starts every fight by killing everyone very quickly.