When i say master, i mean complete all side-quests, fully level up your characters, and obtain all possible weapons, abilities, accessories, armour, items etc. Personally i would say that VII was difficult to master, mainly because of the time spent trying to get a Gold Chocobo, and receiving the rare items including the Desert Rose.

However, Final Fantasy XIII is my most difficult to triumph. Obviously it is a linear game up until you reach Gran Pulse, so if you miss a treasure sphere, you are trumped straight away and have to start again. Well, usually the items from early on are available to buy later on, however there are one or two items unavailable more than once.

Post-game, and once the C'ieth stone quests are finished, trying to fully level up the Crystarium for every character, and obtain every item, accessory and weapon (all 8 weapons upgraded twice for all 6 characters) in the game becomes quite tedious. It is the most frustrating game in the series to fully complete 100%, including the time it takes to farm all the Oretoises to gain the sufficient Gil and catalysts to transform every accessory and weapon to it's highest level.

So for me ... Final Fantasy XIII