How have most people here been dealing with Thundurus? (My word, I hate that thing.)
Prankster Thunder Wave and Taunt are aggravating for sure, so I think neutralizing those threats is top priority. Here're a few Pokémon I think can counter Thundurus pretty well:
Excadrill would give Thundurus a headache. The possibility of Hidden Power Ice is a non-issue for Excadrill, as it'd do neutral damage against it. Plus, its Rock Slide can do some serious damage to Thundurus. And then there's its obvious immunity to Electric type attacks. Thundurus using Taunt against an Excadrill is not the smartest move, either.
Rotom-Wash is another good counter for Thundurus, as it's immune to Thunder Wave and takes 0.5x damage to Thundurus' most common moves. Although a Hydro Pump from Rotom-W would provide neutral damage, it'd still do a decent amount. Taunt isn't as much of an issue for Rotom-W, either.
Mamoswine is immune to Electric type attacks thanks to its Ground typing, and can do a good amount of damage with a super effective Ice Shard.
Weavile, although not immune to Thunder Wave, can also do a decent amount of damage with an Ice Shard. One running Icicle Crash would obviously do more damage.
Mega Venusaur counters non-Hidden Power Flying Thundurus well, too However, like Weavile, it's susceptible to getting paralyzed. As such, it's not as good a counter as the first three. But a Sludge Bomb from M-Venusaur can still do a decent amount of damage to Thundurus. It shrugs off Thunderbolt damage thanks to its Grass typing, and also shrugs off HP Ice damage thanks to its Thick Fat ability.
Thundurus definitely provides headaches if your team isn't prepared for it, though.
