Whew. This is an extremely late answer...
But you said you were prepared for a slightly opinionated answer, so get ready.
-SHADOW MEWTWO-
Shadow Mewtwo is honestly broken. I know you're probably thinking... "Tyo, S-Mewtwo is frail as heck, he only has 480 HP!
Is that so?
Have you forgotten he can restore his own HP?
Recovery
That's right. Not only are his moves wide-ranged, very powerful, and offer him essential immunity to grabs, he can compensate for his HP drawbacks by using recover right in your face. Just hold and release A.
Then, we move on to...
Overpowered Synergy
Did you know that Synergy Bursts are number based? This controls how fast the S-Gauge fills up. Most have a Gauge size of 150-200cc. (CC is the unit system for the Gauge.) S-Mewtwo has a Guage Size of 100cc. Do the math.
And don't forget the incredible power + range of his burst.
Good supports
A (bit more than a) few good supports to team up with...
Croagunk + Sylveon. It's best to use Croagunk to guard yourself when you want to recover.
Magikarp + Pachirisu. Same reason as Croagunk. You could also start a combo when Magikarp lands...
Dragonite + Victini. Victini is OP, offering All crits. Just be aware of the long charge time.
Cubone + Diglett. Both of them can use an attack that will guarantee your burst connects.
Frogadier + Eevee. Use Eevee and your attacks are officially even more broken.
Whimsicott + Jirachi. Whimsicott is a good choice, you can heal and be invincible at long range.
Umbreon + Espeon. Espeon is another support to help sustain you + remove negative status.
Phew... I'm tired. I'm sorry that you weren't able to get an answer for so long. But if you see this, thanks for reading. I hope this answer proves useful to you. And maybe others as well.
Also, I'm unable to provide good teams for team battles. After all, they're just 3 1v1s. Sorry about that...