I think Landorus is better, because of it having greater stats than Tornadus and Thundurus.
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Have Thundurus and Tornadus in your party and go to the Abundant Shrine off Route 14. Head toward the open part in the Abundant Shrine. Be sure to bring plenty of Ultra/Timer Balls. For Thundurus you'll have to trade with White.
Thundurus is in White only. Tornadus wanders around after talking to a certain NPC, just like thundurus
you first get tornadus and thunderus then you have to get a special amount of dream orbs to get the extension for landorus
Well its a bust if you only have access to one game. You must have Thundurus from White Version and Tornadus From Black Version in your team. Once you do, head to Abundant Shrine. It is on the map above Black City or White Forest, depending on your game. Go up to the shrine at the far North of the field and talk to it. Then a Kyogre and Groudon and Rayquayza moment will happen between the three pokemon. Thundurus and Tornadus will struggle and Landorus will settle it. Then you can catch it.
In Pokemon black/white you must have all the badges and catch tornadus/thundurus and get someone to trade the other legendary( ex. if you have white, get someone to trade for a tornadus). Than goto the shrine with both the legendary pokemon in your backpack and landorus will come out of the sky. Save then fight. Source: me got him a long time ago.