Trap sin takes the crown easily. Fire druid will struggle due to fire immunity, not only in cs, but in getting to A4 hell effectively.
Trap sin on the other hand is one of the easiest chars to level up, has a great deal of safety built into her kit (fade) and requires little gear to kill enemies. This is because she comes with 3 elements: lightning (lightning sentry), fire % physical (death sentry's explosion). Do yourself a favor and throw on some crushing blow and put a point into kick to clear out bosses (such as diablo) faster
General tips on an assassin:
Get trap claws from Anya and consider farming cows in the first few days of the ladder reset to get decent socketables & runes, your death sentry is gonna do a lot of work there
