Fang has the best stats for a Commando, and is a pretty good Sentinel (the only thing that makes Snow better in the Sentinel role is his HP).

Lightning has the best stats for a Ravager and the best mix of offensive and defensive abilities.

Hope gets all the useful enhancements you'll need and gets all the Ravager's most powerful spells.

There is actually a strategy guide that gauges the characters in terms of the stat boosts, skills gained in roles and other factors, and it's really these three that have the most potential, while Vanille falls a bit beneath them. Snow and Sazh, though, are just underpowered in comparison. The battle system is broken, so the characters are actually unbalanced.