Charged TM - Machamp or Gengar

I wait until I have 2 before I Charged TM anything - that way if it goes worse I can at the very least switch it back.

Gengar if you're reeaaally in need of current raid counters but it might get Sludge Bomb. That first Machamp for more long-term usefulness and DP or CC are both better than HS.

well, if I only had one CTM I would spend it on a valuable Mon. I would seriously consider to use it on your 100% Gyarados to try to get hydropump unless you have enough of those already. A perfect Gyarados is cool, the Gengar is mediocre and does not deserve your attention unless you are in urgent need.. I would also consider the Machamp if you are low on those

Gyarados can’t learn Twister anymore, so you’ll either get Hydro Pump or Crunch. Bite/Crunch can be decent for Mewtwo/Deoxys raids if you’re lacking other counters.

That said, Machamp is super useful and will continue to be the best fighting type until Gen 5, so getting a high IV one with Counter and Dynamic Punch should be a priority.

I wouldn’t use the TM on Gengar, you’re not guaranteed to get the move you want and it doesn’t have Shadow Claw.

Gengar does not have a lot of uses. Its best niche use is against Mewtwo with Psycho Cut/Focus Blast (and I suppose Alakazam with the same moveset). Against Confusion it dies too fast and against the other charge moves it doesn't have any resistance to help it. With only a single charge TM available, I think Gengar would not be worth it.

Having two (or more) Machamp with fighting moves can be very useful for fighting normal-types in gyms (and rock, dark, and steel) or helping with Tyranitar or Absol raids. Dynamic Punch is the more desirable charge move, but Close Combat still works well. If you try to use a charge TM on the Counter/Close Combat Machamp then you risk getting stuck with Heavy Slam, whereas if you use a charge TM on the Heavy Slam Machamp you are guaranteed to at least get a fighting-type move. Wait until you have at least two charge TMs before you try to change Close Combat so you can roll again if you get Heavy Slam.

Gyarados with Bite/Outrage is an acceptable counter against dragon-type pokemon if you don't have anything better (strong dragon- or ice-types). Using a charge TM on it will give you either Crunch or Hydro Pump. (The weaker moves Twister and Dragon Pulse are legacy, no longer available, so don't need to worry about them.) Bite/Crunch is a usable counter against Mewtwo or other psychic-types if you don't have Tyranitar. Bite/Hydro Pump is an okay water attacker, but needs a fast TM to get Waterfall to really reach its potential as a water attacker.

1 charge TM is nothing because you might not even get the move you wanted on a particular pokemon. You need like 10 to 20+ Charge TMs but at that point you wouldn't have to worry about which pokemon to use it on because you have too many.

There are a few “automatic” charge TM candidates in this game. Two that come to mind quickly are:

1. A high IV Bullet punch machamp
2. A high IV Fire Blast Tyranitar

Regardless what you get with the TM, you have improved.