The Shadow/Marauder build seem to be the easiest and with the lowest-effort build for GR70. BUT the playstyle is very weird and if you haven't played this build before, I'd not use it.
The second best should be either the Unhallowed Essence (Grenades) or the Shadow's Mantle: Impale build. The Shadows: Impale builds will get an insane buff for patch 2.5 using the reworked Holy Point Shot quiver.
Else you can just play the Classic UE Multishot build or Marauder: Cluster Arrow version.
In my opinion, on hardcore, the Might of the Earth build is the most relaxing one. You have a lot of toughness and damage is easy for GR70 with decent equipment and low paragons.
The Whirlwind builds (Wrath of the Wastes or Immortal King's Call) both work aswell. The IK WW version is more tanky, but your damage output is a bit lower than if you are using the Wrath of the Wastes set.
The IK/Raekor combinations have an insane amount of damage output, but you also are very easy to kill, which is why I prefer not to play those builds.
As Crusader, I'd play the Blessed Hammer build, because the damage is just insane and you should just destroy a GR70 with even bad rolled items.
The Blessed Hammer is really squishy tho, you take a lot of damage.
Else you can just play a Thorns based build (Akkhan/Invoker (later LoN aswell)), which are very tanky and more safe to clear a GR70.