Elementalists are strong, but they're highly mana-costly until you get to Endgame levels, where your high MP and Int growths start to accumate and surpass the mana cost.
Summoner is probably the best choice, but probably not the most time-efficient ones, since a couple of bosses will easily kill summons and cost you your MP. Battle Mages can save MP if they stick with melee and spears, but it requires a certain degree of caution and practice to exploit her sliding melee and mobbing skills. Witches get decent AoEs, but the damage output doesn't scale well at endgame levels, and you'll often find yourself running away waiting for cooldowns.
There's really no be all end all class that's easy, strong, AND durable.