Latios honestly should be A+ rank. It's a major staple in the meta with its amazing combination of massive power and utility with Defog. It has a fair amount of unpredictability, and you really can't be comfortable checking it with any Pokemon except for Aegislash and Clefable in this meta, it can muscle through all its other checks one way or another, even Bisharp assuming you can predict the Suckers. Depending on the set it can do incredibly well vs almost all of S and A ranks, and the fact is just because Aegis beats it hard doesn't excuse it for being just A rank. There are definitely Pokemon in the A+ rank like Azumarill and Cube, while they deserve their A+ I wouldn't say those are Pokemon better than Latios, and I really wouldn't want to stick Latios down in the same ranking tier as something like Greninja or Mandibuzz... the point here is that he really stands out compared to the other A rank Pokemon as being way better than them, and better than even some stuff in A+. He really should be moved up.
I've yet to have a latios not die horribly to my tyrannitar. It eats draco meteor and then kills or maims with pursuit. You didn't even mention tyrannitar as a counter when it's one of the best. (I should specify I use a specially defensive support ttar)
Personally, I think Latios is fine in A. It's great, but not THAT great. Especially with how good dark is offensively this gen, and how weak latios is to pursuit.