Assassin is technically the DPS king HOWEVER...
...they need to be behind (or to the side of) and almost in direct contact with their target to do the best damage so when you factor in high end raid mobs (outputting insane dmage AoE attacks) rangers start to look a lot more survivable without giving up a whole lot of DPS in the bargain.
In addition, it's kinda hard to get behind the mob if you are soloing!
("hey you... Mr. Mob... look over there... what is that"?)