Since this is a question about dominant weapon/armor configurations it is assumed that you're already finished with your Capture weapon and that you've already completed your sphere grid using a Triple A` weapon, so neither of those abilities are going to be considered.
Also note that since abilities cannot be removed your best bet is always getting a 4-slot drop that has the most costly abilities already on it.
You definitely want armor with Break HP Limit, since that's the most expensive you'll want to fight monsters that drop 4-slot weapons that already have the Break HP Limit ability. This is best obtained from Dark Bahamut because he may also apply Auto-Protect or Ribbon to the armors.
I break with the greater population here, and suggest that you do not use Auto-Remedy, Auto-Potion, or Auto-Phoenix on your armors, as money for the monster arena will be a factor late game and these abilities only increase the cost of battles (they also shorten the time though because you do not have to take turns to esuna/cure/life your party members, so take my comment with a grain of salt.)
So, armor abilities listed in order of importance:
- Break HP Limit (obtained with the armor)
- Auto-Haste (80 Chocobo-Wings use the Machea trick)
- Auto-Protect (obtained with the armor or 70 Light-Curtains overkill Fafnir for 40)
- Master Thief (Rikku's armor only! 30 Pendulums bribe Ultima Weapon for 99)
- Ribbon (On your First Strike character only! 99 Dark Matters you get these from the Monster Arena Trainer)
- Auto-Regen (All other characters! 80 Healing Springs overkill Kottos for 40)
You definitely want weapons with Break Damage Limit, but you'll only want to use weapons obtained from Penance because those weapons have a hidden critical hit chance of 10% versus the 3% chance of most weapons.
Auron, Wakka, Kimahri, Tidus and Rikku's weapons:
- Break Damage Limit (obtained with the weapon)
- Evade & Counter (1 Teleport Sphere bribe Barbatos for 20)
- Magic Counter (16 Shining Gems bribe Puroboros for 36)
- Gillionare (Rikku's weapon only! 30 Designer Wallets you get these from the Monster Arena Trainer)
- First Strike (On your Ribbon character only! 1 Return Sphere they are a rare drop from Barbatos so try to save one of these)
- One MP Cost (Your other 2 characters! 20 Three Stars bribe Behemoth King for 14)
Yuna and Lulu's weapons:
- Break Damage Limit (obtained with the weapon)
- Magic Booster (30 Turbo Ether bribe Exoray for 30)
- Magic +20% (4 Supreme Gems bribe Halma for 20)
- One MP Cost (20 Three Stars bribe Behemoth King for 14)
EDIT:
I initially thought that Celestial Weapons had a bonus modifier on all damage done by the weapon, it turns out it's a penalty modifier. The penalty modifier uses a RATIO
of either CurrentHP / MaximumHP
or CurrentMP / MaximumMP
in an equation like this:
(10 + 100 * RATIO) / 110
As you can see if RATIO
is less than 1 this actually reduces the damage dealt. The only exception to this is Auron's Celestial Weapon which looks like this:
(130 - 100 * CurrentHP / MaximumHP) / 60
So when Auron's HP is above 70% this is a penalty modifier, but when Auron's HP is below 70% this is actually a bonus modifier. That being said when Auron's Strength stat is maxed out he will do 99,999 damage to anything even without the modifier, and since his Celestial Weapon has disappointing abilities it should be traded in for the dominant weapon recommended above.
Weapons are a bit more complex, the problem is that the Celestial Weapons have additional damage calculations, so a significant amount of work has to go into constructing a weapon that will have only moderately stronger damage. The real benefit from the reconfiguration is the abilities on the weapon. So because Tidus and probably Lulu's Celestial Weapons are so close to the weapon that I would suggest for them I recommend retaining their Celestial Weapons.