Flutter Mane's blistering Speed and Special Attack along with its powerful unresisted STAB combination exerts an oppressive amount of pressure on most teams, making it one of the premier offensive Pokemon in the format.
Mystical Fire: Mystical Fire scorches the common Flutter Mane check, Scizor. It also heavily disrupts enemy special attackers.
Power Gem: Power Gem gets a bit of extra damage against Fire-types like Ogerpon-H and Incineroar, which can be relevant when using Tera Stellar.
Substitute: Calm Mind + Substitute is a classic duo that greatly assists Flutter Mane's setup attempts, allowing it to set up in-front of Thunder Wave Porygon2. Even without Calm Mind, it can save Flutter Mane from devastating uses of Thunder Wave from the likes of Archaludon.
Taunt: Taunt is hugely disruptive against a variety of support Pokemon, such as Swampert, Ting-Lu, and Porygon2.

Stellar Tera Blast: With Tera Stellar, Tera Blast can be some good insurance against Pokemon with unpredictable Terastallization options, but the Special Attack drop doesn't gel well with Booster Energy.
Draining Kiss: When using extremely bulky Flutter Mane variants, Draining Kiss can be paired with Calm Mind to keep it around much longer. However, Tera Fairy is highly recommended when using this, as Draining Kiss is otherwise very weak.

Booster Energy: Flutter Mane's incredible Speed is put to good use, outspeeding almost everything in the metagame with the major exceptions being Booster Energy Iron Bundle and the very rare Choice Scarf Dragapult.
Focus Sash: Focus Sash patches up Flutter Mane's poor physical bulk, giving it new revenge kill chances.

All of these EV spreads are designed to give Flutter Mane a Speed Boost with Protosynthesis.
4 HP / 108 Defense / 140 SpA / 4 SpD / 252 Spe with Timid: The Defense investment usually allows Flutter Mane to survive Sucker Punch from Adamant Chien-Pao. To require Adamant Chien-Pao to get a critical hit to OHKO Flutter Mane with Sucker Punch, an EV spread of 4 HP / 124 Def / 124 SpA / 4 SpD / 252 Spe can also be used.
116 HP / 4 Def / 252 SpA / 4 SpD / 132 Spe with Timid: This EV spread outspeeds Jolly Roaring Moon and survives an opposing Flutter Mane's Shadow Ball without Terastallization. In addition, Life Orb Mimikyu's Shadow Sneak has a fairly low chance of OHKOing, and Iron Bundle fails to KO with the combination of Hydro Pump and Freeze Dry.
4 HP / 252 SpA / 252 Spe with Timid: If using Focus Sash, this simple spread is best for maximizing Speed and power. Note that this spread's vulnerability to priority makes it a poor fit for Booster Energy variants.
236 HP / 252 Def / 20 Spe with Bold: This extremely defensive spread allows Flutter Mane to survive Tera Dark Life Orb Jolly Chien-Pao's Sucker Punch without needing to Terastallize itself, while still outspeeding Choice Scarf Jolly Landorus-T and Gouging Fire after Booster Energy.

Tera Dependency - Low: While Flutter Mane benefits from Terastallization, it's generally flexible enough to go without it.
Stellar: Flutter Mane boosting all of its offensive moves once gives it good offensive flexibility, especially since it retains the important immunity to Extreme Speed.
Fairy: Tera Fairy makes Moonblast a potent sweeping move, while reducing damage against Chien-Pao's Sucker Punch and Mimikyu's Shadow Sneak.
Water: Tera Water is handy for giving a resistance to Steel, Ice, and Fire, giving improved matchups against Scizor, Ting-Lu, Chien-Pao, and Chi-Yu.
Electric: In addition to resisting Bullet Punch, Tera Electric protects Flutter Mane from the dreaded Thunder Wave and Glare!
Fire: Tera Fire gives Flutter Mane an offensive and defensive edge against Scizor, though the weakness to Urshifu-R's Aqua Jet is quite unwelcome.