A well developed, fully equiped Hammerdin is by far the most capable single character in the Diablo game. There are other characters that in combination out perform a solo Hammerdin in PvM (Player vs. Monster). However, for single character to single character, the Hammerdin is the top killer in the game.
Does this mean that the Hammerdin is the easiest or even the most productive character? No.
In terms of easy: the Hammerdin is quite tricky to play. The hammers generated by the Hammerdin require some practice to aim. In fact, attacking a stationary target can be near impossible until one learns the tricks of where to stand in order to have the hammers exit properly to hit.
In terms of productive: perhaps the fact that EJ requires all Intermediate and Advanced Bardo trainees to play each characters will answer this. Yes it is productive. But so is each of the other characters. In fact, there is something(s) to be learned from each of the characters. And playing just one is unlikely to give you the bardo training you need.
This graphic demonstrates the damage that a Hammerdin can generate without the use of the Concentrate aura.
When the Concentrate aura is employed the damage jumps dramtically. With extra skill points generated by equipment and "skillers" a Hammerdin can easily do 8000 and even 10,000 points of damage.
The principle skill for a Hammerdin is the Blessed Hammer. This skill has two synergies: Vigor & Blessed Aim.
Because Vigor is a synergy to Blessed Hammer it is maxed with 20 skill points (more with equipment).
Because Blessed Aim is a synergy to Blessed Hammer it is maxed with 20 skill points (more with equipment).
Concentration is the aura to use with Blessed Hammer. Other auras do not up (enhance) the damage of Blessed Hammer the way that Concentration does.