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Landorus-T would do well in Gen 1, but he'd need paralysis support to survive. A QuakeSlide SD set with Hyper Beam or Explosion, which can OHKO/2HKO most of the meta after 1 boost, sounds great. However, he'll get OHKO'd/2HKO'd by SE special moves. Tauros, Jynx, and Starmie could outspeed and shut him down with Blizzard 1v1 without being paralyzed, but it has 90% accuracy.
Not much to say about other top Gen 7 threats imo. Lele could be almost as good as Zam with just the Psychic typing, but Zam has better tools: higher Special, Speed, Recover, and TWave. Lele does have better physical bulk and could be a good matchup against Starmie and Zam with TBolt, so at least it's not completely outclassed.