Second vote for Witch Doctors definitely for a first class just leveling to 70. It allows for a lot of room for error and to get a feel for the game, as well not as gear dependent.
From there you have a lvl 70 that you can now power level other classes if you choose to (since the game really shines at max level). Also you earn Blood Shards which you use as currency to gamble on for specific items. Some users do this to gear up other classes since you can play the well geared character and spend it on the not so well geared. Albeit this might be a bit slower than actually just playing that class.