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.
I play HC, and while there are many builds that are available to that can clear GR 70 early in the season, I suggest that you try Shadow Assassin on DH for season 10. It's very HC friendly in that it has incredible survivability while at the same time having enough damage output to clear a GR 70 with modest gearing and paragon.
The Marauder is this season's Haedrig's gift for demon hunters and it can walk through a GR 70 with all the proper gears. Super tanky too.
I strongly recommend NOT to play with the Manald Heal without experience first, it can walk through a GR70 in 3-4 minutes with trash gear but an inexperienced player can die at any moment with it on GR70.
I did GR80 on a HC DH after about 20 hours total of solo play last season using the M6 set. I was plvl'ed to 70 (so saved like 4 hours), but did not have haedrigs gift so I had to farm starting from normal, through about T8 then moved on the GRs.
Considering GR70 only needs like 1/4th the DPS of GR80, and the DH haedrigs in S10 is marauders, I'm sure the entire leveling + GR1-70 process can be done as DH in like 10-15 hours total.