This is the list of checks fo Landorus:
Taking from here
http://www.smogon.com/forums/threads/ou-checks-compendium.3532549/
(Can switch on it)
Gengar (be ware of Psychic), Gliscor (with Knock off, Toxic and specially Ice Fang), Latios (hard check, use Draco metro/Surf), Talonflame (be aware of Psychic and specially SR), Celebi (be ware of Knock off), Charizard-Y (One fo the best, even if ti's a Fire type), Latias (one fot he better checks), Rotom-W, Gyarados (not Mega evolve if possible), Tornadus-T /specially Assault Vest), Chansey (be aware fo Knock off), Blissey (one of the few reason it's viable is because of Lando-I), Mega Latios/
Mega latias (one of the better checks),
Cresselia (with Ice Beam), Mandibuzz (be aware of Focus Blast, specially with SR on), Zapdos (soft check without HP Ice because immunity to amin STAB and Heat Wave not beign enough), Bronzong (be ware of Knock off).
Of those, only Cresselia and defensive Mega latias are counters of Landorus. The rest are only checks of Landorus.
(Can't switch on it, but can defeat it 1vs1)
Keldeo (outspeed + STAB Water), Azumarill (STAB Water with Aqua Jet + can take 1 Earth power after SR), Thundurus-I (with HP Ice), M Gardevoir (can take 1 Earth power and Hyper Vopioces OHKOes after SR), Mamoswine (Ice Shard), Starmie (outspeed + Water Move), Kabutops (in rain, please use him only in rain), Kingdra (specially in rain), Kyurem-B (be ware of Focus Blast), M Sceptile (only with Leaf Storm and specially Hp ice), Weavile (strict revenge killer with Ice Shard), Thundurus-T (with HP Ice) and Froslass (outspeed + Ice Beam).
The best way fo dealing with Landorus is using a moderate bulky Pokemon that resist its main STAB (if you notice, all the main checks resist Earth power except Chansey, who is the bane of special attackers anyway).
The best option is Creeselia with Ice Beam, but it really only fits on defensive teams. In more offensive teams, the best Pokemon to use are the Lati@s, followed by Celebi, Charizard Y, Rotom-Wash and Normal Gyarados.
The main problem of Lasndorus (as some people said) is that he has so few direct switch-ins, the majority of them (actually all fot hem bar Cresselia) not being sure switch-ins, causing many teams to have to revenge kill Landorus to defeat it.
Apparently, the problem of Landorus-I is bigger when people advance in the ladder. In the mid-lower ladder, generally switching into a Pokemon with Earth power if the Pokemon that Landorus-I was sent out is heavily affected with it, generally works. However, when advancing in the ladder, more teams are able to make good predictions and will try to use the coverage move that hits your most rpobable switch-in, with makes witching more difficult, specially if you currently has one available answer to Landorus.
And of course, Landorus doesnt have 4MSS. With Earth power, Psychic and Focus Blast it can 2HKo 95% of the metagame and with Knock Off, it can deal with some fo the few things that can switch on Lando (mainly Celebi, Lati@s and Chansey)