Starting this off...
Tyranitar@Choice Scarf
Trait: Sand Stream
Evs: 252 Spd / 252 Atk / 4 Hp
Jolly/Adamant Nature
-Crunch
-Pursuit
-Stone Edge
-Earthquake
Scarf Tyranitar is a great addition to team 2, for many reasons. Scarf Tyranitar has fallen out of favor with many recently, but it always packs a wallop for those unprepared for a fast sand-godzilla.
Team two is set up to go sand well, as celebi is a staple on sand teams and heatran is also beneficial in sand. This also all but forces team 1 to go rain, to support Thundurus-T.
Tyranitar can deal great damage to all three members, while out-speeding them as well (except pirouette form) with an Adamant nature (assuming speed rounds up, I think it does). It obviously cleanly KOs Thundurus-T with Stone Edge, KOs non-pirouette Meloetta with Crunch and deals a fairly clean 3hko to Metagross with Crunch, and 2hkos with Earthquake.
Tyranitar will also provide team 2 with some speed, something it sorely lacks currently with Celebi and Heatran as the members. Tyranitar also pairs well with Celebi's Thunder Wave, as if Meloetta gets paralyzed, Tyranitar can trap it with ease.