Best any scenario Mewtwo counters?

You would be best served just powering Tyranitar. Nothing is better all around and either your raid group is strong enough to take a FB Mewtwo or it isn't. It would be a large expenditure for things that still won't outperform Ttar by much if at all.

Now, if you're drowning in TMs like some of us are it wouldn't be too bad to flip a Gyrados or Scizor etc. All I'm doing is looking to add mons 7-9 that will come in before (or after) the first group. My raid partners average no more than one Tyranitar per trainer so we will likely struggle with many C movesets. I'm not going to change that with a few Gyrados and I doubt others will either.

The almighty Dragonite may be a viable option, as your "any scenario counter".
But then, being good at everything is the same as being good at nothing.
Gyarados is a less Dragonite. Even with double dark moves it doesn't deal as much damage as the genuine dragon.
Tyranitar has wonderful performance against non-FB Mewtwo. Against a FB Mewtwo, Lugia is what tops the list from pokebattler.

I'm picturing two type of teams to deal with Mewtwo:
1. Diversified team: 4x Tyranitar + 2x Lugia. If you don't have that many T Tar, Houndoom will also do, but T Tar with double dark moves is preferred.
2. No Brainer team: 6x Dragonite. If you don't have that many Dragonite, Gyarados with Bite + anything will do.

I don't like bugs or Gengars as they die super fast.

Feraligatr is already terrible against Confusion 2500 CP Espeons, not to mention Mewtwos.

I wouldn't spend the dust to power them up, but I wild caught a couple high level magikarp, evolved and tm'd them to be B/C Gyrados. I plan to augment my ttar with these. I also have a shadow ball alakazam, and a couple s/fp houndoom. So, I can field 9, or so, relatively high attack stat mon that will be type effective into mewtwo. That is my plan, anyway, and I probably don't have enough time and resources to put a different plan in place before mewtwo hits.

I hope the bugger is, at least, halfway easy to catch.

Personally my top 2 cp mons are 3200cpTtar B/SE so using those plus 2750Ttar B/C, 3015 GyradosB/C and 2 3000cp DniteDT/O. Then Lugia DT/Sky Houndooms,Scizors,more Dnites on my 2nd team. Opinions? So I guess TTAR or Dragonite to answer your question.

I have three above 91IV and above 30 level Ttars and a fourth that is close. They all have BSe as their movesets. I have a maxed out 96 IV Bite Crunch Ttar and an 80 IV at level 21 with BC.

Should I TM the fourth BSe Ttar to BCrunch with Mewtwo coming?

I only have one larvitar candy.