Magearna To S
Magearna has an excellent type, with a lot of resistances and immunities.
His ability allows him to be a dangerous snowball for an end game sweep.
Their offensive coverage is immense and unpredictable. BoltBeam + Z-Focus, STAB-based, and still have valid options for hitting counters and checks such as HP Fire for M-Scizor and Shadow Ball for Marowak-A.
Your Stats are all solid except Speed, while Trick Room and Shift Gear correct this easily.
Assault Vest is a solid tank that does not lose offensiveness, and still has Volt Switch for momentum.
Not putting perfection in this mon, Magearna has problems with stall as it does not break Chansey and is vulnerable to Dugtrio, but stall is a distinct atm.
Zygarde To S
Okay, your Attack base is not huge, but the main factor here is the
limitation on team build that Thousand Arrows causes.
Zygarde has several options of sets, SubCoil, Band, DD + Z-Outrage (which allows a powerful STAB without lock in Outrage), have features to hit checks, either Iron Tail for Tapu Bulu, Toxic or Z-Outrage to Tangrowth. Still has Extreme Speed and a good bulky, with or without investment. Also, Grassy Terrain don't affect Thousand Arrows.
Even naming Zygarde and Magearna to S, they can not be compared to
Landorus-Therian.
Take all the ORAS qualities and add powerful Z-moves that have excellent coverage with your STAB Earthquake from a high Base 145 Attack.
Only two weaknesses, immunity to Spikes and Webs, Decent stats and Intimidate, often being the answer against any physical hit, immediately becoming a threat.
Landorus-T has access to an incredible varied of sets.
Defensive Pivot, where he is the most solid Stealth Rock user of the metagame, and access to U-Turn for momentum.
Landorus-T use not only HP Ice, but Protect to scout a possible Z-move from another Landorus-T, shows how this Pokémon is a massive presence in the metagame.
The metagame lacks answers to its offensive variants, be it Z-Edge or Z-Fly.
I still mention another sets; Stallbreaker (SD + Smack Down @ Earth Plate), and others not as popular as Scarf, Lead Explosion and Special / Mixed lure, to talk how Landorus-T is complete and varied, and excellent glue for any team.
Currently there is no reason not to use Landorus-T, except maybe Stall and Trick Room. Your 40%+ usage also needs to be mentioned.
Landorus-T exceeds the qualities of an OverUsed Pokémon, and deserves
a suspect go to
S+.