Tornadus
Tornadus-Therian is the superior of the two, primarily due to its Ability, Regenerator. Despite lower attacking stats, Regenerator allows it to run Life Orb with little to no drawback and allow it to scout or sweep. The Incarnate form really have nothing that great going for it that the other can't do. Its Ability is completely wasted as it lacks any useful Status move and its coverage is decent at best. A possible move-set for the two could be:
Tornadus-Therian (M) @ Life Orb
Ability: Regenerator
EVs: 4 Atk / 252 SpA / 252 Spe
Hasty Nature
- U-turn
- Hurricane
- Heat Wave
- Dark Pulse
Tornadus (M) @ Sitrus Berry
Ability: Defiant
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Knock Off
- U-turn
- Taunt
- Acrobatics
Thundurus
Here the Incarnate form reigns supreme. Access to Priority Thunder Wave and good moves and stats make it a dangerous foe to face. Therian is mostly the same but lacks the Ability, making it less versatile, despite having higher attacking stats. That being said, both are really good and shouldn't be underestimated.
Thundurus (M)
Ability: Prankster
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Thunder Wave
- Volt Switch/Thunderbolt
- Hidden Power [Ice]
- Sludge Wave/Focus Blast
Thundurus-Therian (M) @ Choice Specs
Ability: Volt Absorb
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Hidden Power [Ice]
- Volt Switch
- Focus Blast
- Thunderbolt
Landorus
Both the Lando's are monsters, doing slightly different things but being about equal in it. Therian have higher attack and different ability in Intimidate, making it an excellent tank, wallbreaker, scout, lead or Choice user. Incarnate on the other hand has better Speed and Sp.Atk, along with sheer force and a lot of moves that allow it to abuse that ability, allowing it to run physical, special or even mixed sets. Overall Therian is a bit more useful thanks to its capacity to wall common threats, but Incarnate is more unpredictable.
Landorus (M) @ Life Orb
Ability: Sheer Force
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Earth Power
- Psychic/Sludge Wave
- Hidden Power [Ice]/Knock off/Stealth Rocks
- Focus Blast
Landorus-Therian (M) @ Leftovers
Ability: Intimidate
EVs: 252 HP / 252 atk/def(depending on your preference) / 4 SpD
Adamant Nature
- Stealth Rock
- U-turn
- Earthquake
- Stone Edge
So in short:
Tornadus-Therian.
Thundurus-Incarnate.
Landorus is equal.