Running about and quickly killing open world mobs --> Bard (MCH close)
Killing a bunch at once or something tougher / when you don't have space to kite --> SMN. (Takes longer to ramp up damage and has weaker filler but also has a tank and/or knockback/stun.)

After that, probably DRK / WAR.

Largest SMN advantage is queue time. It can always just swap to SCH, especially until Mind splits from Intelligence on gear (~lvl 42). Not solo content, but hugely worth considering.