Landorus-I @ Smooth Rock/Life Orb
Sand Force
Lonely
4 HP, 252 Att, 252 Speed
- Sandstorm/Stealth Rock
- Earthquake
- Hidden Power [Flying]
- Smack Down
I'll be the first to admit this is a somewhat gimmicky set, but it helps Landorus deal with opposing Flying types especially well, and would be useful in a sand team. Sandstorm is there to start off Landorus's boost, or if you have other pokemon with Sandstorm or rely on Hippowdon or Tyranitar for their Sand Stream, you could replace it with Stealth Rock. Earthquake is Landorus's main attack, being boosted by both STAB and Sand Force up to double its normal power, with Smack Down acting as Landorus's Rock type attack. While Rock Slide and Stone Edge would be stronger, this set's focus is to counter common Defoggers such as Crobat and Skarmory by taking away their Flying typing with Smack Down, either forcing a switch or forcing them to take a super-effective Earthquake. This could also be useful on several pokemon with Levitate that might switch in expecting Landorus to EQ or Earth power right off the bat. HP Flying is added due to Landorus's unfortunate lack of a Flying physical attack (excluding Fly), and its only Steel attack, Iron Tail, has shaky enough accuracy not to be worth using. The purpose is both to allow Landorus to hit physical walls and to surprise a Chesnaught or Breloom trying to come in and absorb your other attacks.
If running Sandstorm, it could be wise to have Landorus hold a Smooth Rock to allow the sand to last longer, but otherwise you'll want to give him Life Orb for an added power boost (bringing EQ to 260 power in Sand, Smack Down to 87 power, and Hidden Power up to 117). Lonely nature sacrifices some of Landorus's physical Defense to allow it stronger EQs without giving up the power in its special move, but if you're confident that Hidden Power will kill what it needs to even with a hit to its power, Adamant could work as well.
The biggest downside to this set is that it relies heavily on Earthquake to deal damage. Hidden Power is only useful on things with high Defense that would otherwise resist Earthquake and Smack Down, particularly bulky Grasses like Chesnaught, Tangrowth, or Venusaur, and occasionally to power through bugs that aren't part Poison, Rock, Steel, or Flying (non-Mega Pinsir or Heracross in particular come to mind), and Smack Down is weak enough that it's really only useful to remove the opposing pokemon's immunity to Ground so you can threaten it with a powerful Earthquake or to hit something that's 4x weak to it that would otherwise resist EQ (which is essentially only Bug/Flying types). The other downside is that it's somewhat weaker without Sandstorm support, but Earthquake is still powerful without Sand Force boosting it, and Hidden Power is unaffected by Sand Force anyway.