Choice Thundurus-T is fairly one-dimensional. With its STAB Thunderbolt and Hidden Power backed by a Choice Specs boost, Thundurus-T is more than capable of ripping holes in the opponent's team, provided it doesn't get caught out with the wrong move in the wrong situation. Thunderbolt is self-explanatory, but Volt Switch might need more explanation, considering the rarity of such moves in VGC. The premise behind Volt Switch is this: although Thundurus-Therian hits hard, in a metagame where everything is either EVed to take Dragon Gem Draco Meteors, some Pokemon need softening up before Thundurus is ready to knock heads. Furthermore, Volt Switch hits almost as hard as Thunderbolt does, making it a safe move to throw out in the beginning of the game. The choice of Hidden Power largely depends on how well your team takes on Fighting-types or Dragon-types, but due to STAB, Hidden Power Flying is naturally the more powerful option. Hidden Power Ice, however, gives you the edge against other Thundurus-T that chose Hidden Power Flying, and it is powerful enough to OHKO Yache Berry Garchomp at about the same rate as Scald burns (and we all know how often that is), which is something normally only Ice-types such as Mamoswine and Abomasnow can claim to do. In the same situation, Garchomp cannot OHKO Thundurus-T without resorting to the inaccurate Stone Edge.
As for the third slot, Grass Knot is the best that Thundurus-T's rather shallow movepool has to offer, but it can be useful in situations where Thundurus-T is left alone against Tyranitar or Gastrodon. It might also be wise to study up on the weights of some Pokemon before haphazardly locking yourself into Grass Knot. A prime example is Rotom-W, which only takes about 25% from a 20 Base Power super effective Choice Specs-boosted Grass Knot. If you have trouble memorizing weights, pay attention to the 'slamming' animation that most Pokemon make when entering the field.
This analysis has so far listed a few examples of Thundurus-T's power. It should be noted, however, that those examples are the work of a Choice Specs boost, which means you won't be getting the same mileage out of Choice Scarf. Choice Scarf is still a great option though, as it outspeeds Choice Scarf Tyranitar and Landorus-T, Latios, neutral +1 Volcarona, and other such Pokemon that Thundurus-T might have interest in outspeeding. This makes Choice Scarf Thundurus-T most useful in revenge killing, especially if you have Fake Out users or Pokemon that can easily soften up the opposing team.
Thunder is a viable option when using Thundurus-T with Politoed, especially because of its 30% paralysis chance. Thunder should go over Volt Switch however; because of Tyranitar and Abomasnow, there will be times where Thundurus-T just can't afford to risk them switching in and causing Thunder to miss. A Timid nature could be used, but is mostly only good for outspeeding other Thundurus-T, and in Choice Scarf Thundurus T's case- Timid Volcarona after a Quiver Dance. The power drop usually isn't worth it because of this, as Thundurus-T barely ekes out a 2HKO against 4/0 Timid Volcarona with Hidden Power Flying, and it'll obviously be in a tight spot against Volcarona that opt for a bulkier spread. Another thing to note is that, when using Hidden Power Ice, you should use an EV spread of 4 HP / 252 SpA / 252 Spe to avoid wasting EVs.
As mentioned earlier, Politoed is an excellent partner for Thundurus-T for many reasons. First and foremost, Thundurus-T's Volt Absorb takes advantage of Politoed's Electric-type weakness, making it easier than ever to preserve your precious frog. Secondly, Politoed provides speed control and Helping Hand support, boosting Thundurus's destructive prowess. Finally, rain brings with it Pokemon such as Kingdra and Ludicolo, which are more than happy to deal enough damage for Thundurus to be able to clean up later.
Like Politoed, Cresselia has Helping Hand and speed control in the form of either Icy Wind, Thunder Wave, or Trick Room, the latter of which can actually benefit Choice Specs Thundurus, as it might find itself outsped by the aforementioned Speed demons. In fact, if you're feeling cheeky, you can even heal Thundurus-T with Cresselia's Thunder Wave. Other anti-Trick Room Pokemon are also appreciated because of Thundurus-T's relative frailty. Amoonguss is a great example of this, as it can outspeed some, if not most Trick Room attackers with a Sassy nature and put everything to sleep to stall out Trick Room or, in a more general sense, it can keep the heat off of Thundurus while the latter plugs away at the opposing team.
Tyranitar is an odd option, but it — along with Excadrill — appreciates a STAB Thunderbolt working against nasty Water-types, especially Politoed. Excadrill and Thundurus-T can also serve as a "fast mode" with Excadrill spamming Earthquakes to its heart's content while Thundurus-T takes no damage and spams its own STAB moves. Fake Out users are also much appreciated, especially those with Feint, which allows Thundurus-T to nab some unexpected KOs or simply ease prediction. Examples of these are Infernape, Hitmontop, and Sableye. Other useful Feint users include Scizor and Heracross, both of which take care of Tyranitar, Cresselia, Abomasnow, and sometimes Mamoswine, all of which are threats to Thundurus-T.