Hello again R3BEL!
For competitive battling...
Zekrom:
Zekrom bolsters a whopping 150 base attack, alongside decent bulk which can it to at least become 2HKO, not to mention it's slightly superior typing, capable of switching into electric types at ease.
Singles: If this is Sword and Shield, with the introduction of dynamaxing, it is without a doubt that DMAX Zekrom, alongside a life orb will become a serious threat in monotype teams. It functions similar to Dracozolt, except possessing greater bulk, speed and a higher attack stat, alongside being able to hit any Pokemon, regardless of their ability. This seriously threatens Rotom users if Zekrom uses Earth Power, or Mimikyu in general. Outside of Sword/Shield, life orb/choice band/choice scarf are all viable, allowing it to both wallbreak as a bulky attacker. The only issue that I can see for Zekrom is its weakness to fairies and other dragons, which means that it would most likely be a 2HKO. Smogon also suggests that you could run hone claws + substitute, taking advantage of passive mons such as Blissey to set up a +6 Attack - monstrous.
In doubles, nothing changes much. It functions as a viable wallbreaker or a dynamax sweeper. It extremely enjoys its 150 base attack, an unprecedented base stat which is superior in comparison to other legendaries/mythicals.
Landorous Incarnate:
As we mentioned previously, here, we realized that Landorous-Incarnate can be a fearsome Sheer Force + Life Orb sweeper. Sheer Force ignores the 10% chip from life orb, allowing Landorous to effectively gain a +30% damage. This isn't to mention its ability to mixed-attack and great typing, being only weak to electric and x4 to ice - which just requires you to be vigilant enough to not switch Landorous into a Milotic. Landorous was banned from OU at some stage, and they mention the reason being: (it's here fyi)
Only 2 non-Ubers Pokémon in the game are capable to achieve this major feat : Cresselia, whose viability in the OU metagame is close to non-existent despite its very high defenses. Mega-Latias, which is not even distributed in-game and therefore, is unavailable.
Thus, Landorous (I) was a HUGE threat in OU, having little to no checks in the format, allowing it to dominate as a proficient mixed attacker.
Same probably goes for doubles.
Rayquaza:
Now things only go even crazier - Rayquaza. I won't mention its mega form, or else it would be too obvious who the winner is here.
The only time I have ever seen Rayquaza been used was during 2016 Worlds VGC, where Wolfey Glick used it to effectively become champion. He ran a Sashed Rayquaza too.
In X and Y, Air Lock was a crucial ability to have, being able to switch into Kyogre and Groudon and weaken their attacks, Origin Pulse/Water Spout and Eruption. Some people, who couldn't obtain a Rayquaza for whatever reason, began using Golduck to purely counter these sun/rain teams. Imagine the impact Rayquza would have, being able to substitute a mon like Golduck.
Onto its stats, although its bulk seems decent, its EVs are most likely going to be used to maximise its attack and thus, Focus Sash or Life orb would be viable. Its niche 150 atk and sp.atk allows it to be a mixed attacker, far more dangerous than Landorous, especially with the fact that Rayquaza can extreme speed knockout low HP mons, making it a fearless late-game sweeper than can throw anyone off their tracks if they aren't prepared for it. Combining it with its high BP movepool, Dragon Ascent, Draco Meteor, Extreme Speed and Outrage, it can literally cripple an entire team with stealth rocks set up to inflict some chip into the mons switched in.
To prove its superiority, Wolfey Glick won worlds in 2016 and came close in 2017 with Rayquaza, where he used it on numerous occasions.
Honestly, I believe that it is without a doubt that Rayquaza is the best of the three, even if I may be a bit biased being a Wolfey fan...
However, Zekrom and Landorous are equally frightening, but Rayquaza is Rayquaza.
