Show me something eternal
Just re these onesRises:
1. Vivilion: C --> B-: This is a real mon, unlike the turds that currently reside in C that give your opponents free wins. Viv has one of the best boosting moves in the game, ultra accurate sleep powder and STAB hurricane. Its speed tier is definitely good for essentially a spore user as well. Outside of that, you have a ton of options as your filler slot: Bug buzz, sub, tailwind off the top of my head. This mon doesn't need nearly the level of support other guys in this tier need to really dent or flat out win games, and does not belong in C.
5. Espeon, Ninetails, B- --> B: Screens is underrated here, its gotta be at least as good as rain imo, and these both have good STABs and useful utility outside of their obvious niche.
Latias: B --> B-: Choiced sets that hit like wet paper unless its a super effective attack won't sell me. Its mega version outclasses it and as a result it should drop
Gigalith: C --> UR: Can't imagine why this was ever ranked, lol
Vivillon is practically only seen with the moveset Hurricane, Sleep Powder, Substitute, Quiver Dance with Leftovers. It's 4th most used move has 98.2% usage, and Leftovers has 97.5%, it really is a one set mon. It often requires support though, most Viv teams will have mons that carry Rock Tomb, Icy Wind etc as Viv needs to get its foot in the door, the first Sleep Powder, to get anything going. In addition, due to it's poor offensive coverage it can find it's self struggling with some common mons, mostly steel types, and especially so when they're combined with either Koko or Fini to make sleep far less reliable. I think these are genuine negatives for it.
Screens can be kind of dead weight against certain kinds of teams and also somewhat costly, so I don't think they're being particulalrly underrated. Especially as options like Koko also exist for dual screens. Espeon and Ninetales-A are fine where they are right now.
Latias is actually more common than it's mega form. Z-Psychic, Rocky Helmet and as you mention Choice Specs are pretty common among the handful of Latias used in higher ladder teams. Also, the choiced sets aren't the kind that are looking for an ohko, it's on a decently bulky mon, so it's more about trading efficiently than being an all out aggresive pick.
I don't think it's fair to say the mega outclasses it when they often do different things plus the opportunity cost of using it as a mega, though not huge, is still there
Gigalith was ranked because it saw some strong placements and served a genuine niche over Ttar and Hippo as a sand setter for Excadrill. Most notable it's access to Gravity and Explosion let it get Excadrill in safely and allow it to plough through what could have otherwise been checks with z-EQ. However it has seen very little usage as of late, and it is certainly no longer being considered a factor fo the meta, so UR may be right for it.