I've played assassin the last hardcore seasons. Started out with it. It's one of the best characters to start out a season with, due to burst of speed. Also, wake of fire is incredibly overpowered in normal and nightmare, and can make progress in hell too. Although lightning sentry and death trap becomes mandatory and op by then. They make progress viable in hell mode.
Assassin is not very gear dependent, but as any character, needs some kind of gear to make it through hell. Fun to play, fast, and can be one of the best characters in the game. Trap is the only build i've ever played.
The elemental druid however, is much more difficult to make work. Needs a lot of good gear, but fissure is kind of op early game. In hell, you need very good gear to make it viable. But it can be a very powerful build.
The funniest druid i know how to play is feral rage+fury wolf, with ribcracker. Eth upped ribcracker deals insane damage, and is a crazy boss killer due to the 50% crushing blow. I once had it drop early in nightmare, i think from andariel, it can drop that early. And decided to make a druid at once. It just kills everything so fast, leech is high when feral rage is active so survivability is good, also makes you run fast. Upgraded, it's level 49+7, only level 56, 63 str and 35 dex, low requirements, so it can be used that early. Highly recommended and lots of fun. Just wrecks everything.