Basic overview of Balanced Hackmons:
-Wonder Guard is illegal
-OHKO moves (Sheer Cold, etc.) are illegal
-Pure Power and Huge Power are illegal
-Shadow Tag and Arena Trap are illegal
-max EVs in every stat
-any move and any ability on any Pokemon

I'll add some stuff here when I have a chance.

-Sturdy Shedinja is broken...
-Unless your opponent has a Mold Breaker/Gastro Acid/Prankster-Toxic Pokemon, which are fairly common in Hackmons.
-That's why I use Magic Coat on my Shedinja.
-One time I was against another Magic Coat Shedinja, and the other guy Struggled himself to death as I switched out at the last second, to use my Shedinja as death fodder later on.
-SCARF IMPOSTER BLISSEY.
-You get to use Ubers too.

The issue with (and reason I have stopped using Shedinja) is that in the face of damaging weather, it is completely useless. Its not so much that Shedinja is hindered by factors as much as the fact that you're playing 5+death fodder-6 with said factors.

One time I was against that somerandomanimecha guy, and we thought each other's Shedinjas had Toxic when they really had Mirror Coat.

Needless to say, he switched out to his Mold Breaker Deoxys-S as I got out my Slaking.

I like to use a combo of Regigigas with a Ghost type carrying Shadow Tag. Regigigas typically is a prime target for imposter Blisseys, but my Regi knows all normal/fighting moves, so when I switch to something... Say Chandelure, I can set up entirely, and then proceed to destroy with Shadow Ball+Aura Sphere

What's the consensus on Slaking vs Regigigas, that is which one is better?

I prefer slaking

I like Slaking too

I like Regigigas because of his much better special bulk. Slaking may have insane HP, but that doesn't matter if he lacks the special D to support it.

Plus special attackers are much more common due to the banning of Huge power.