Best ranged phys DPS: PR (Phoenix Ranger)
Best melee phys DPS: MR (Master Rogue)
Best direct-damage spellcaster: Ele(mentalist) [<lol @ mentalist reference]
Best summoned units: Dark Summoner (DS, DSumm)
Best tank: RG (Royal Guardian)
Best Healer: High Priest
Best mobile ranged DPS/Best at getting around: Battlemage
Best mobile tank/melee DPS/Best at getting around: Savior

There are 2 best at getting around in my opinion. Battlemage goes anywhere on the map, but has cooldown and cast time. Savior on the other hand has a range (like 2-3k or something) but no cooldown and a tiny cast time (Savior Blink is like Aura of Knowledge/Gods blink, but 2-3k range and anytime you want for 50 mana)

And anyway it's all opinionated unless there's a clear rigged unit, in which case you'll love and hate it. Love because it's, well, rigged. Hate because, well, it's rigged.
But anyway, it's all up to you to decide what YOU think is the best overall.