All the classes are suited for different Purposes.
Guardians,
-> really good in soaking up damage, but are not suited for dealing damage like the dedicated classes.
Shadows,
-> Really high damage, plus the chain type attack helps clear mobs really fast. Mostly kit revolves around solo play.
Champions,
-> A balance between offense and def, and have nice aoe attacks. My preferred unit type for melee.
Mystics,
-> AOE nuke galore. but takes some time to prep (cooldown locked)
Rangers,
->High single target dps, great boss killers. They tend to have stuns as well.
Healer,
-> a supporter thats welcome on any team.