If you're fighting the original creations in the monster arena (Specifically earth eater) you'll probably want a few different sets of armor. One set of 'ultraproof' with the different status (especially confuse proof and deathproof)

But usually it doesn't matter after so long a while. Auto Haste, Auto Protect, Auto Shell, and Deathproof would just about be the ultimate fortune sphere gaining armor. (ie. earth eater)