The Most Rare-Candy-Worth Legendary

Disclaimer: this is not a question but rather a sharing of personal view on the game.

I've briefly talked about legendaries in a previous post (https://pokemongo.gamepress.gg/q-a/lets-talk-about-legendaries) and how they would change the meta. The conclusion was most of them are very powerful and gonna top the specialist list. But which should be given priority to invest rare candies/dust/TMs in?

The first one that came into your head is probably Mewtwo. Yes, this bad boy has the highest CP of all Gen1-2 Pokemon. But CP isn't that important anymore, so what exactly does Mewtwo offer aside from CP?
You might think he'll be a Psychic specialist with double Psychic moves. Yes, he can be, but he can be much more. With the Super Effective buffed from x1.25 to x1.4, a Psycho Cut/Focus Blast Mewtwo can be a really viable Fighting attacker! Remember, Mewtwo's base ATK is almost 100 units higher than that of Machamp. Simulation results from pokebattler suggest that this kind of Mewtwo is nearly as damage-efficient as Counter/Dynamic Machamp when fighting Blissey, but almost twice the KD (Kill-Death ratio)! And Psycho Cut is much more dodge-friendly.
And there's more for Mewtwo. Shadow Ball is another amazing charge move (as powerful as Outrage). Therefore, he can fight some other Psychic legendaries as well (with itself resisting Psychic attacks) like Lugia. The Psycho Cut charges energy really quickly and he just launches Shadow Ball one after another.
In conclusion:
a) Psychic Mewtwo: Best Psychic Specialist, great generalist as well;
b) Focus Blast Mewtwo: Wonderful solution to Blissey/Snorlax;
c) Shadow Ball Mewtwo: Best Counter to other Strong Psychic Type legendaries;
d) Hyper Beam Mewtwo: best Charge TM recipient. And then, I see the three birds being discussed a lot recently, especially Zapdos. Let's talk about Zapdos first.
a) Zapdos is super cool and one of the most iconic Pokemon in the franchise. But you may need to have a second thought. Zapdos's best moveset is Charge Beam/Zap Cannon, while Raikou's best is Thunder Shock/Wild Charge, which yields 9% higher circle DPS than Zapdos's, which makes up for the little bit less base ATK. Besides, Thunder Shock is more dodge-friendly. Therefore, personally, I would save my candies for Raikou.
b) Motlres. This bird is the real deal. Fire Spin/Overheat? Checked. Highest ATK of all fire types? Checked. Double resistant to Grass? Checked. It is an enhanced Charizard. Moltres out-tiers Entei in almost every perspective except for fighting an Ice type. But are Fire attackers that useful in the current meta? The remains a question.
c) Articuno. Some think that his base ATK is underwhelming (less than 200, lower than Jynx). It is, but it is still a great ice attacker, which will prove to be useful when Dragonite is introduced as Raid Boss. Jynx dies too fast and Lapras is less damage-efficient. Articuno needs love. Ho-Oh and Lugia? Ho-Oh has great stats. He is another 4k+ CP monster, 2nd highest CP of all. But if you look at the moves of Ho-Oh, you'll feel bad for it, as well as Lugia in this sense. Such a waste for fantastic base stats. Unless Niantic decides to give them new moves, I won't dump any rare candies on them. No one wants Poor Mew, Celebi, or Suicune. These three are so much weaker than their other legendary friends, though they are really nice collections to have. Still, if you don't even want to waste candies on Ho-Oh/Lugia, you sure won't use any on these three.

Afterwords:
I doubt that the buddy distance for legendary is 5km. It'll probably be more, such as 10km, given the exclusivity and the ever-growing greediness of Niantic. All the reason why rare candies must be spent only after careful thinking.
And as always, any discussion is welcomed.

Cheers,
bioweapon