1. I still think mega sceptile is the best water stopper in the tier. You're using the argument that it takes up a mega slot and while it does, it doesn't excuse the fact that Sceptile takes care of water types better than any of the other mons you listed. Whimsicott has no offensive presence and usually doesn't run giga drain. Shaymin doesn't have as good offensive stats to take care of those water types, Rotom-C is only limited to one move bc its scarfed and its usually volt switch so it can't excatly stop an opponent from sweeping. Roserade faces a similar problem to sceptile where it's weak as hell if giga drain and setup fodder if leaf storm, but doesn't 4x resist water attacks. Tangrowth is super slow and is can't even check special attacking waters like Blastoise well, whereas sceptile has no trouble taking it out.
2. The lose/lose thing isn't exactly true. Even if Suicune have rest, it can't do shit back, as giga drain sceptile risks a 3HKO, so it has to rest right after it wakes up and risk sleep talk again. it's more of a stalemate than anything else.
3. This is not the case if it is running knock off 0- Atk Mandibuzz Knock Off vs. 0 HP / 0 Def Mega Sceptile: 57-67 (20.2 - 23.8%) -- guaranteed 5HKO.
4. Opportunity cost isn't the main thing that should tell the viability of the pokemon. You're blowing it way out of porportion and saying "opportunity cost this" "opportunity cost that" and again, that shouldn't be the only thing telling how good something is. btw, the thing that sceptile offers that other grass types don't is STAB Dragon Pulse, which beats the dragon types that most grass types can't beat even in their dreams, and good Coverage in Focus Blast, which allows it to beat steel types. This is why I think it should stay B+.
1. Offensive variants of whim do run Giga, Rotom-C can run lefties, Scarf, or Specs(!) that all beat water types, shaymin gets a LO and a 120 BP move that can drop spdef, Tangrowth (both defensive and LO variants) fucking shit on water types, Roserade gets utility in sleep and spikes. I'm sorry but you are both blatantly ignoring the fact that mons have other sets and that Sceptile is not some fabled water type slayer. if it was as godlike at stopping waters as you made it out to be, it would be A. It's not. Other mons do the same job that it does with similar or better effectiveness without costing a team its mega slot.
1a. Addressing the coverage arguments made at the bottom of point 3 here because they don't deserve their own point and frankly aren't that good. I'm gonna go ahead and give you a mon by mon breakdown of grass types that compete with Sceptile to get my point across.
Whimsicott: gets Fairy coverage which is infinitely better than drag coverage, and gets U-Turn for momentum, prankster, support moves, leech seed
Rotom-C: Electric coverage (beats flying types), support moves in Wisp/Pain Split, momentum with VS, can run Scarf, Specs, and Lefties giving it increased versatility.
Tangrowth: Regenerator, actually beats water types, support moves like Seeds/Knock/Sleep Powder, better movepool than sceptile, can run LO/Specs/AV/Lefties giving it increased versatility.
Shaymin: Bulkier, gets LO, coverage in Dazzle/Psychic/EP/HP Fire to choose from, can rest off damage, better STABS
2. Suicune quite literally loses nothing by continuing to click rest and sleep talk vs sceptile. Suicune either can threaten scept with a burn, get another CM, or lose 36% of its health, all of which translate to Suicune winning end game.
3. Quite literally every mandi does and should run foul play to check mence and prevent it from being setup fodder for stuff like gatr.
4. Opportunity cost is a major reason to recategorize megas and weigh them more harshly against other mons in the rankings. When you have the option to just slap a rotom-c on over Sceptile and then add Aero to blanket check half the tier and check offense then you have quite literally zero reason not to do it. I could see your argument if Sceptile wasn't one dimensional, weak, and frail, but better mons exist for doing that job. You haven't given me or anyone reading this thread a reason to not weigh the opp cost argument other than saying "thats not how it should be." Until you provide evidence showing why opp cost is a false philosophy when judging megas (along with providing concrete evidence for the rest of my post) then I still firmly believe that
Mega Sceptile should drop to B.