My hot take of 2025, and potentially an interesting discussion point.
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Is Latias an S tier Pokemon?
In my opinion, Latios is the best Pokemon in the game and Latias is
probably the second best.
Latios is better. Obviously. The 130 SAtk is markedly stronger, takes KOs earlier, and the extra power enables weaker coverage moves (most relevant is probably Hidden Power [Fire] 2HKOing Careful Skarmory).
That said, I feel like Latias can viably be swapped with Latios on basically any occasion where Latios isn't using Hidden Power [Fire], or the incredibly rare Earthquake or Memento. This seems to have started with people experimenting with Latios -> Latias as the Colbur Thunder Wave supporter on Clefable/Reuniclus teams, where the better SDef makes Latias an attractive option as the mono water resist. With Spike support and a back-up Psychic type, for example, Latios often isn't taking advantage of its 130 SAtk to the maximum anyway. Its abuse of typing, speed, and residual damage support is what makes the Pokemon so good and you can get away with less power.
Increasingly we have seen Latias begin to compete with Latios for its other sets also. Dice and others are prominent believers in Specs Latias - it can certainly be argued that Specs Latios power is overkill in many situations and that trading some of that SAtk for bulk + Keldeo security is meaningful.
When it comes to Latias' unique traits, beyond the slight bulk increase, attention is immediately drawn to Healing Wish which appears to play greater and greater roles in BW OU every single year. This is unique to Latias among the twins, and has been used extensively on weatherless hyper offenses alongside Conkeldurr, Volcarona, and so on, most recently abused by Sergio Aguero on a winning Circuit run. A couple of years ago, I would have said "Latios fits on every team archtype", but as hyper offenses begin to smartly implement Healing Wish more and more, I feel that Latias is possibly even a better option on a large chunk of weatherless teams. Healing Wish has utility beyond hyper offense - again, dice is a believer in Specs Healing Wish as a glue on some sand balances, where the ability to heal partners such as Tyranitar, Lando-t, or Excadrill in the lategame can be match winning.
The key questions to me that help to work out how to place Latias are as follows:
What is the point of a viability ranking? If a Pokemon sees mid-low usage due to competition, but is incredibly incredibly good when you do use it (e.g. Latias), where do you place it? In reverse, when a Pokemon sees incredibly high usage due to lack of other options for the slot, but is actually only "fine" in-game (a lot of people would put Jirachi in this category), does its absurd usage influence where you place it? I think Latias and Jirachi are the two most difficult Pokemon to place in the entire VR, and fall at polar opposite ends of this debate.
The second question I think is helpful is:
How much weaker would Latios need to be before it wouldn't be the defacto number 1 Pokemon in the game? Or another way of viewing it would be,
would Latias be #1 in a metagame without Latios? For me, I think Latias is plenty powerful enough to where it plays functionally the same as Latios in most situations. Its typing, speed, moveset, and ability to abuse Spikes all define Latios more than its special attack does, in my opinion, I think an 80 SAtk Latios would be like, B tier? But at 110, with extra bulk and healing wish? Yea, for me thats among the best Pokemon in the game.
tl;dr Latias is the second best Pokemon available to use in the game, is not entirely outclassed by Latios, and there is a strong argument to be made that it is S tier