Hi, I definitely understand the word counter -- a Pokemon that can switch in a reasonable amount of times to the majority if not all of a Pokemon's move set. I'm saying the truth. I can agree on the U-turn targets, though Landorus often runs RP or even Psychic.
Cresselia is a great counter to Landorus-I, but again, outside of countering Landorus-I it's un-viable, which shows that it's having a pretty big impact on stall teams.
Chansey and Blissey are decent checks, but are both 2HKO'd with a little bit of residual damage.
Wrong on both counts. Cresselia is a damn good wall that you shouldn't underestimate. It's near impossible to OHKO and is insanely difficult to break in sun. Blissey and Chansey aren't getting 2HKOed by anything unless you're running max max bold which is stupid. They have Softboiled and Wish; they're the last Pokemon I'd define as "easily worn down".
And yes, I can say that the Landorus user requires perfect prediction as an argument. You're assuming that the Landorus user plays perfectly and that the [insert example Landorus counter here] user is stupid. This assumption has no basis. Anyone smart is going to bait and lure certain attacks. Landorus has U-turn, RP, or occasionally Psychoc. All of which are extremely easy to scout for.
252+ SpA Life Orb Sheer Force Landorus Earth Power vs. 248 HP / 252+ SpD Slowking: 169-200 (43 - 50.89%) -- guaranteed 3HKO
252+ SpA Life Orb Sheer Force Landorus Earth Power vs. 252 HP / 252+ SpD Jellicent: 175-207 (43.31 - 51.23%) -- guaranteed 3HKO
So with Leftovers recovery these are 3HKOs. Additionally, you're assuming Landorus went straight for the EP. Also, Slowking is a solid Pokemon. No need to bash it.
0- Atk Life Orb Landorus U-turn vs. 72 HP / 0 Def Latias: 156-185 (48.9 - 57.99%) -- 95.7% chance to 2HKO
So much for "boned by U-turn".
252+ SpA Life Orb Sheer Force Landorus Psychic vs. 88 HP / 0 SpD Gyarados: 168-198 (47.59 - 56.09%) -- 22.27% chance to 2HKO
You can run a bit more bulk to avoid the 2HKO (which I do) and have a flawless counter.
252+ SpA Life Orb Sheer Force Landorus Focus Blast vs. 248 HP / 252+ SpD Mandibuzz: 166-196 (39.24 - 46.33%) -- guaranteed 3HKO
Where are you getting this 2HKO from? And no it's not a specific counter, it's a generally bulky mon that can handle a pretty decent range of threats.
Gengar is 2HKO'd by HP ice
This is true, but Landorus doesn't exactly go around spamming HP Ice. I'd say Earth Power is the spammed move. Either way, you force Landorus out. With a mere two turns of Black Sludge recovery you can survive one more. Your Gengar should be running HP Ice so it can OHKO Landorus and handle Dragon-type threats.
I'll concede that the bulky Waters and Skarmory are simply great checks, as well as Celebi, but you still have quite the list. A counter isn't something that can beat a Pokemon in every possible scenario -- it's simply one that can take 1-2 hits and respond with something while being able to come in another time.
It's not all about taking hits -- if you can take one hit and KO or force out Landorus it's the same thing as beating it. In fact, you are beating it. Due to its flaws, which Pocket pointed out extremely well, Landorus is relatively easy to force out and check.
@Icecream: Cresselia too can have team support. As can the other Pokemon. Dugtrio says hi. Also Landorus needs to be broken in the context of itself. I know we don't operate in a vacuum but you can't ban a Pokemon for being difficult to handle in a three Pokemon core.
EDIT: oh and I forgot zapdos