The best gear is obtained by opening all the Master Key chests and finishing the game's longest side-quest. The items are as follows:
Master Key Rewards:
•Hoston: Elsydeon & Guardian Armour
•Alina: Valkerye & Prism Dress
•Kruller: Mithril Rod & Crystal Armour
•Zellini: Pixie Whip & Spiked Jacket
Arburus Side Quest Rewards:
•Edessot: Fire Anchor & Light Armour