Final Fantasy XIII is a very linear game. It's on-the-rails not just in terms of the journey, which is filled with a seemingly endless number of hallways, but also in regards to character progression. One would think that this wouldn't be the case with an RPG (least of all in a series as storied as Final Fantasy), but this game changes that formula.
Nevertheless, the game's Crystarium (the substitute for standard level-ups) gives set path for each character, pointing them towards a few potential roles (or paradigms). Even when the game opens up later on, there's no point. Each party member has a specific job they've been forced to develop and that they excel at. Choosing the ideal role for a set character in Final Fantasy XIII takes some practice on the player's part.
6 Lightning: Ravager
As the main protagonist, Lightning is the most versatile fighter in the party. Her melee prowess makes her an accomplished Commando, but it's ultimately the Ravager paradigm that wins out in her arsenal. Her offensive spells pack a powerful punch on their own and skyrocket the stagger meter on any enemy. Even better, players can infuse this magic into her gunblade, enabling various elemental melee attacks to increase the effects. She can then switch to Commando to deal damage or let one of her party members do it.
Keep in mind, though, the Ravager mode is only the best in the overall sense. As was previously mentioned, the Commando paradigm is on par. Which one is best is subject to change depending on Lightning's equipment and which chapter she's on. That said, if players want the Army of One ability (which is basically the "win" button for every battle), then they'll have to invest in the Ravager role.
5 Snow: Sentinel
This musclehead usually has the most health of any party memeber. Because of that, he can obviously take a serious beating and still stand. This makes him perfect as the group's resident Sentinel, which is basically akin to a Tank class.
Snow has several abilities to attract enemy attacks and absorb damage intended for other party members. At the same time, moves like Steelguard ensure that he doesn't get slaughtered himself. Plus, some of the more advanced defensive moves (i.e. the ones that heal players and damage baddies) are exclusive to Snow until late in the game. It's clear the designers intended this to be his main role, because it's the best way to win his Summon battle with Shiva. Snow may not boast much brainpower, but he always strives to protect people. In that sense, the Sentinel role is perfect.
4 Sazh: Synergist
Don't let the guns fool you. Sazh is a Synergist, a role which essentially exists to beef up one's allies. He's got a decent amount of health and generally tries to stay out of danger. This theoretically keeps him alive long enough to bestow buffs.
Throughout his progression, Sazh unlocks several abilities strengthening the party's stats. Bravery and Faith are the most prominent ones. He gets these and other synergistic spells at lower levels than the rest of the characters. Sazh may be a deadbeat dad, but he takes good care of the team.
3 Hope: Medic
Easily everyone's least favorite character, Hope's dismal physical strength matches his short stature (and his grating whininess). He's no good at melee attacks or even ranged strikes with his boomerang, and he can't take much abuse from adversaries. Despite these weaknesses, he works well as a mage. Hope comes equipped with a variety of spells, casting them in rapid succssion to easily stagger an enemy. However, he works best as a healer, which makes him the ideal Medic.
His curative magic is easily the strongest in the game, and can pull players out of several tight spots. Even better, he unlocks these restorative spells earlier than most of the other characters. His only competition here is Vanille, but her destiny lies along a different path. Basically, Hope should stay out of the way and make sure his allies don't die.
2 Vanille: Saboteur
It's ironic that the one trying to lift people's spirits is also the one to bring down an enemy's. Vanille is the go-to Saboteur of the group from the beginning, having quick access to abilities like Deprotect and Deshell.
As with other specialized characters, Vanille earns more advanced spells at lower levels. Sadly, some Saboteur moves are blocked off to her for whatever reason, but she is the only one who can inflict Death later on. That's got to count for something.
1 Fang: Commando
Though she comes in much later than the other party members, Fang's basic gist should be obvious to series veterans. Her staff makes her akin to the Dragoon class of old. This handy tool lets her juggle enemies in the air, wholloping them with melee attacks while keeping them staggered.
As a result, Fang specializes in the Commando paradigm. Even after Lightning's had numerous hours to build up her roles, Fang trumps her in this one. She hits hard and prevents the enemy from doing much in retaliation. Not to mention, the paradigm's ultimate Highwind ability is exclusive to her. This deals multiple devastating blows to unsuspecting foes. The only downside is that it also knocks them out of their stagger state, but that's a small price to pay.