XIII-2 is really not so bad, I think the average JRPG fan can enjoy that one.
Anyway I spend the most time in a single playtrought with FFX, for me it is very enjoyable and I loved a lot of the side quests. It is not my favourite one but the amount of content/great music/and a very interesting level up system made me spend 200-300 hours in it. I had one time a holiday of two weeks and I only played that game, people were shocked when they saw me again because I lost more than 10kg. I survived on rice crackers ^^'.