I've been studying and battling VGC for a while now, so here are some effective leads I've seen on other teams, that I've used myself and that work well in theory.
Mega Kangaskhan + Azumarill
The goal here is for Kang to Fake Out one of the opponents, allowing Azumarill to get a Belly Drum and wreck house. Other Fake Out users work here, but Kang has the bulk to stay in longer than most others.
Mega Salamence + Clefable / Clefairy
Mega Metagross / Heatran + Togekiss
Azumarill + Amoonguss
The premise here is called redirection. A Follow Me / Rage Powder user redirects attacks aimed at their partner to allow them to use a setup move, usually Dragon Dance or Substitute.
Mega Charizard Y + Venusaur
Charizard sets up Sun, activates Venu's Chlorophyll, and the wreckage begins.
Politoed + Ludicolo/Kingdra/Mega Swampert
The same as the previous strategy, but with Rain and Swift Swim.
Heatran/Rotom-Heat/Arcanine/Talonflame + Gastrodon
Use Gastro's Storm Drain to suck in Water attacks aimed at your Fire-type associate.
Mega Kangaskhan + Smeargle
The infamous Khan Artist lead. Kang uses Fake Out while Smeargle hopefully hits both opponents with Dark Void. Make sure to have a Focus Sash to garuntee a Dark Void.
Terrakion + Whimsicott/Weavile
This is commonly known as TerraBeat. This must, must be used at the start of battle. Simply have Terrakion's partner use Beat Up on his Justified… instantly netting a +4 Attack.
Rotom-Heat + Landorus-Therian
DisQuake. Rotom gets to spam Discharge and Lando gets to spam Earthquake. Simple.
Here are some less conventional – but still effective – strategies.
Heliolisk + Mega Sceptile
This one's a doozie to pull off. Scarfed Heliolisk outspeeds everything, and can thus Electrify any attack aimed at Sceptile. This makes attacks activite Scept's Lightningrod, rather than inflict damage.
Klefki + Mega Metagross/Mega Kangaskhan/Terrakion/Breloom/etc.
This is a really gimmicky strategy, but a funny one nonetheless. If Klefki can successfully get up Safeguard, you can Swagger your physical teammate and give them free boosts without Confusion.
Perish Trap
Perish Trap deserves a whole section to itself, since generally it requires a whole team built around it. Here's the strategy in short: Mega Genger, Gothitelle or both are necessary. Set up Perish Song, then alternate Fake Out, redirection and Protect to keep your Shadow Tag 'Mon alive. More experienced players can easily take out your Shadow Tagger with something like Hydreigon, but it's a really fun strategy and a really annoying one to face.
