Honestly up to you at this point.
I liked the entire trilogy so my point is gonna be a little biased so...
- FFXIII is honestly a mix between the hardest game in the saga or the second hardest game in the saga. Mainly because of the fact that you got Haste, Bravery and Faith on complete dial and learning how to stagger becomes easier the more you fight an enemy and know when they are open and the more you know about how to keep the stagger gauge afloat. The towns not being in the game so to speak never really bothered me because mainly, I just never gave a crap about towns. They honestly done nothing in past games but be window dressing and you will barely go to them again past plot or if they connect to somewhere you should care for.
The linear aspect of the game is a shame though which is something XIII-2 and especially LR gets rid of.
- XIII-2 is the easiest game in the saga...until you reach DLC content. That doesn't mean the game isn't hard at times(If you don't grind and fight certain enemies), it's just far more lenient with the changes to the battle system. While you stupidly have no more Haste, Bravery and Faith on demand(only certain monsters have it, either as spells or Feral Link), you no longer pose when you change paradigms for the first time, the battles remain pretty quick themselves and the DLC bosses will easily give you some real challenge with the newly designed battle system. And making the best monsters in the game can easily take you lots of time...but sadly, the game can become pretty easy since at times, you don't really need to debuff enemies at all because a good chunk of them is stupid brittle.
Yeah and the story is the weakest by far, the characters themselves aren't too bad though. But the game itself is pretty non-linear with how you can approach it(you can literally leave an area entirely, head into a different one and then come back to that area exactly before you left it.)
- LR can definitely be stated to be the hardest in the series. The game is easily the most non-linear of the series and it's VERY easy to die alot and screw up in the early game due to the massive discrepancy of certain enemy types as well as learning the battle system.
It's story is also pretty so-so but again, I honestly liked the character progression though. Word to the wise, there's an ability that stops the flow of time for the game(since the game is on a time-limit, no spoilers for that), DON'T SPAM IT. Despite everyone and their mothers telling you otherwise, you got way more time than you think you do and wasting that makes it harder at times to easily recover in battle(since the EP has some useful abilities tied to it). Mainly, it's better to actually see the battle system of LR before really getting into the game because it's that different(damn near heading into KH territory).