Gengar is a decent...ish check. It can't switch in at all into Stone Edge or Psychic at all, but between Earthquake/Earth Power, Sludge Wave, and Focus Blast, it can switch easily. SubDisable sets can completely cripple Landorus if it doesn't obtain a speed boost. Other than that, keep Gengar away from Landorus.
This thing is still an absolute monster, though. Even if Sandstorm was nerfed, Landorus+Tyranitar for Offense is fun. Tyranitar can start up the sandstorm and Landorus can use a Physical Set to work off of the weather. Even then, Landorus can play coy and use a Special set to throw everybody off, which was half of what made Landorus so dangerous last gen: Unpredictibility. He is still on par/just below the par with some of the most offensive Pokemon (though he'd much rather have more Speed, even after the slower metagame, mixed offenses is still good) and can still hold his own, especially with the OHKOs he nets. After all this time using it (since even before my post here), I would still argue that Landorus has near zero counters, with Goodra being as close as it gets. Azumarill+Landorus can fix up most of the problems since Landorus resists Poison and is immune to Electricity and Azumarill resists Water and Ice. Now throw in Ferrothorn for the core and gg.
Just a bit more thought off the top of my head while I sit here bored in class.