Lando-I has 3 main sets actually One has the standard EarthPower, Psychic, Knock off, and a filler move of your choice for which you can choose HP-ice, Focus Blast or Sludge Wave. Rock Slide is an option I see so rarely I don't think it's worth mentioning.
The second set has 3 offensive moves and a Calm Mind, which could mean probably having leftovers as the item.
The third is the Rock Polish. I believe this is the most threatening set, as it bypasses the speed check, outspeeding almost all scarfers on the meta, which means that in terms of speed control it can only be dealt with Talonflame.
In general it is a great pokemon that has multiple uses as a special attacker with utility moves. Several pokemon can work as checks, but the problem is that Landorus-I works so well as a late game sweeper that it can be virtually unstoppable.
A counter I can quickly think of is M-Scizor, if specially invested (probably a check, I don't have time now for calcs), and that's due to Lando's poor defenses.
Need to play the suspect test for more data and to make sure. Right now, I'm not having too much difficulty with it, and I have used it only in one team, which didn't prove to be a very good one, so I don't have a problem with it staying.
edit: Damn Vlado for ninja