Whew... this is not gonna be fun to research. Thanks OP.
As we all know, the best counter for a Gigantamax Pokémon is a Dynamax Pokémon with a superior typing. Let me just run through all of them.
NOTE: Giving proper explanation apparently exceeds the 12000 character limit which I often do. So, enjoy the mild and simplified explanation I give.
> A physical attacking bulky set resists both of Butterfree's STAB and lets you strike back.
> Sumwun will be disappointed, but Togekiss resists G-Max Chi Strike like it was a poke and can hit back hard with Max Airstream and Max Starfall.
> Its dual typing is an excellent defensive type against G-Max Gengar and Max Darkness is a nice way to do damage and help your teammates hit harder.
> Maractus is a surprising choice, but Water Absorb and Max Overgrowth together make it almost invincible against Kingler. Roserade is another option that is vulnerable to Foam Burst but it works.
> Aegislash's powerful STAB and offensive stats help it break through Lapras's fairly good defenses.
> Aegislash with Max Knuckle (base Sacred Sword) hits Snorlax very hard while Hawlucha hits just as hard with Max Knuckle (base High Jump Kick) while avoiding Max Quake.
> Excadrill isn't poisoned by G-Max Malodor, outspeeds and can hit very hard with Max Quake.
> Chandelure's 145 Special Attack helps it outspeed and obliterate Corviknight fairly easily with Max Flare.
> Aegislash has an excellent defensive typing against Orbeetle and can hit it extremely hard with Max Phantasm.
> Roserade outspeeds Drednaw and Maractus doesn't which is why Maractus is currently ruled out. One Max Overgrowth is usually enough to kill it.
> A counter for Coalossal is Water-type, has a powerful special Water STAB move, and outspeeds Coalossal despite the effects of Steam Engine. Inteleon fulfills all three perfectly.
> Rotom-Frost comes late, but you asked for the best counter so here it is. 105 Special Attack with Max Hailstorm against a x4 weakness is especially destructive.
> See Flapple/Appletun's description. You're immune to G-Max Sandblast due to Levitate so don't worry.
> Centiskorch's pitiful 60 Defense gets absolutely obliterated by a physical Max Geyser from Barraskewda.
> G-Max Smite's confusion ailment is more threatening than the actual damage in this case, plus STAB Max Steelspike hits very hard.
> Aegislash will not work due to G-Max Snooze almost always OHKO'ing it. Lucario hits physical Max Steelspike extremely hard but Excadrill hits harder and doesn't resist G-Max Snooze.
> Alcremie is usually a 2HKO with Max Steelspike, which happens to be the reason I use Aegislash for so many of these battles.
> Copperajah's defenses stinks and Chandelure has 145 Special Attack. It's obvious why we're using this.
> Duraludon has a pitiful 70 HP and 50 Special Defense. One Max Knuckle (base move Aura Sphere/Focus Blast) and essentially Duraludon is dead by Lucario's hands.
There you go OP. There you have all the answers for all 19 possible Gigantamax Pokémon (20 if you count Flapple and Appletun as different) and what Pokémon counter them. I hope you have a nice day.