Seconding Mega Latias to be ranked, probably somewhere within the subranks of B.
It can reliably switch into most pdon variants and defog any rocks pdon has laid down or deal decent damage to pdon while remaining healthy throughout the match due to having reliable recovery (unlike gira-o which can get easily worn down). Lati can switch into mixed pdon sets that carry powerful special fire attacks and even ice coverage due to its typing and solid special bulk, whereas megamence cannot as effectively. Sets with dragon type moves, especially dragon tail or swords dance + dragon claw, which can pressure and break through mega lati, are uncommon.
Outside of checking pdon its mixed bulk and typing lets it switch into certain threats such as deoxys-a, mewtwo or pogre and threaten them out or damage them heavily with draco meteor or grass knot, although it often cannot counter them by itself. It can also ko slower mons that have been worn down or are weak to its attacks with its decent speed and offensive presence (unlike gira-a). It is also useful vs webs teams as it ignores sticky web and can defog it, and can outspeed and ko certain webs breakers/sweepers such as kyurem-w and sd ray, as well as psyspam teams due to its resistances and having the bulk to take an ice beam from psychic terrain abusers and hit back hard with draco meteor, though it often cannot deal with them alone due to deo's sash or mewtwo's bulk.
Finally, Lati can run Role Play to copy Shadow Tag to allow it to switch out of goth, giving it a niche over support defog arceus which just get trapped. In addition it can run Toxic for status spread and chip damage, Grass Knot for reliable damage on pdon and pogre or Refresh to remove status, allowing it to defog in front of support mons relying on status to deal damage and rockers that rely on toxic to pressure defoggers (without needing to subject itself to sleep unlike Rest users).
Obviously it has lots of flaws (takes up the mega slot- opportunity cost, threatened out by ghosts, darks and fairies, deals negligible damage to steels, fairies or specially bulky mons (if not running toxic for the latter two or grass knot for pogre) allowing them to set up all over it, stat drops from draco meteor mean it loses most of its offensive power after attacking with it, either weak to status or goth (or both) depending on its fourth move). It requires a lot of support to function optimally, and it's quite one-dimensional, but imo its ability to consistently switch into the best pokemon in the tier and stop it and its rocks in their tracks while having the option of being able to escape goth is enough to at least put it on the list somewhere in B.