So I looked into a couple of potential Kangaskhan versus Gengar scenarios to try to decide for myself whether to run Sucker Punch or Crunch, and what kind of Gengar can counter or be countered by Kangaskhan. Calcs were done assuming both are using +Spd natures, all evs into speed and their respective attack stats, and both have yet to take damage. I used stages to simulate the Parental Bond being half as strong.
MegaKangaskhan versus Gengar
Kang Sucker Punch vs. Geng Focus Blast: +1 252 Atk (custom) Sucker Punch vs. 0 HP / 0 Def (custom): 350-412 (134.09 - 157.85%) -- guaranteed OHKO for Kang.
Kang Sucker Punch vs. Geng SubDisable: Gengar forces a switch, and is guaranteed a free substitute. He also gets a free attack on the switch-in unless Kang correctly predicts the Disable and switches out instead of attacking again.
Kang Crunch vs. Geng Focus Blast: 252 SpA (custom) Focus Blast vs. 0 HP / 0 SpD (custom): 262-310 (74.64 - 88.31%) -- guaranteed 2HKO. 70% of the time, Kang survives with 25% hp at best, and kills Gengar. 30% of the time, Kang kills Gengar without taking any damage.
Kang Crunch vs. Geng SubDisable: -2 252 Atk (custom) Crunch vs. 0 HP / 0 Def (custom): 128-152 (49.04 - 58.23%) -- 96.88% chance to 2HKO. Accounting for health loss from substitute, Gengar loses >50% of his health and forces Kang to switch out. He also gets a free turn on the switch-in unless Kang correctly predicts the Disable and switches out instead of attacking again.
MegaKangaskhan versus MegaGengar
Kang Sucker Punch vs. Geng Focus Blast: +1 252 Atk (custom) Sucker Punch vs. 0 HP / 0 Def (custom): 306-362 (117.24 - 138.69%) -- guaranteed OHKO for Kang
Kang Sucker Punch vs. Geng SubDisable: -2 252 Atk (custom) Earthquake vs. 0 HP / 0 Def (custom): 128-152 (49.04 - 58.23%). Gengar subs against the Sucker Punch. Then he can disable the Sucker Punch and get nailed by Earthquake on the same turn, losing his sub to the first hit and losing over half of his health to the baby. If he also has Focus Blast, he can now use it without getting Sucker Punched, resulting in the next scenario but with <25% health remaining instead of all of his health. Or, predicting that Kang will not try to Sucker Punch again and instead just attack with Earthquake, he can Focus Blast for the next scenario but with a free substitute.
Kang Crunch vs. Geng Focus Blast: 252 SpA (custom) Focus Blast vs. 0 HP / 0 SpD (custom): 320-378 (91.16 - 107.69%) -- 43.75% chance to OHKO for Gengar. There is also a 30% chance to miss entirely, reducing the chance of a clean OHKO down to 30.6%. 69.4% of the time, MegaGengar dies to Crunch.
Kang Crunch vs. Geng SubDisable: -2 252 Atk (custom) Crunch vs. 0 HP / 0 Def (custom): 102-122 (39.08 - 46.74%) Gengar subs against the Crunch, loses the sub to mama and takes damage from the baby. Combined with the loss from using Substitute, he has <50% health remaining. At this point, he must attack or switch. If he has Focus Blast, he can go for the previous scenario. Otherwise, his next best option is Sludge Bomb. (252 SpA (custom) Sludge Bomb vs. 0 HP / 0 SpD (custom): 180-213 (51.28 - 60.68%) -- guaranteed 2HKO )
Both Sucker Punch and Crunch beat regular Focus Blast, but Crunch has a 70% chance of taking heavy damage.
Both Sucker Punch and Crunch lose to regular SubDisable, but Crunch breaks Gengar's substitute and deals heavy damage before being chased out.
Both Sucker Punch and Crunch beat Mega Focus Blast on average, but Crunch usually takes a huge amount of damage first, while Sucker Punch always wins with no damage taken.
Both Sucker Punch and Crunch beat Mega Focus Blast + SubDisable on average, with the same odds.
Conclusion: Sucker Punch is strictly better against any kind of Gengar except for SubDisable from a Gengar that has not yet mega-evolved (note that it is still allowed to carry its megastone), which would give Gengar a free sub and possibly also a free attack on the switch-in. Crunch does not have this issue, but is a far less decisive counter to the average Gengar, especially Gengar that have already mega-evolved or carry Focus Blast. In short, I'm just going to run Tailwind Talonflame and call it a day.