In all honesty, at this point
Mega Venusaur should have been dropped. It's been brought up so many times and there really wasn't any action taken around it.
It's just the idea that if you use it you're losing the opportunity to preserve the Mega Slot and most of time you're thinking, "Why should I use this over Amoonguss?" Fun fact, there's hardly any reason to. The biggest contributing factor that Mega Venusaur has over Amoonguss is it's ability to better checks Electrics, feasible offensive presence, and better overall bulk. Considering Electrics are on a steady decline and the utility that Amoonguss brings in Clear Smog, Spore, Stun Spore, etc none of the other perks that Mega Venusaur has is just usually overshadowed. Then they is the notion that Mega Venusaur is just gets overwhelmed far too easily when relying on Synthesis as recovery in a Sand and passive damage infested meta. The popularity of hazard stack is a huuuuge deal too. It just can't stand up to offensive pressure anymore without completely folding to it and being overwhelming in many matchups. Amoonguss (and by extension Tangrowth) don't have to worry as much about being progressively overloaded with Regenerator usually keeping them healthy enough to consistently check threats throughout the match. This doesn't mean Amoonguss or Tangrowth should go to A- though, B+ is perfect for Pokemon of their caliber, if anything their near the top of B+ if that makes you feel better. So all in all, Mega Venusaur should drop to
B+.
I really think
Tentacruel would be better fit for
C- or possibly lower, rather than C. On paper it seems like such a good mon, but it just seems to fall short a ton of time in practice. It's a seeming good support mon that necessitates a lot more support than it offers. Using Tentacruel usually means that you intend to capitalize on Toxic Spikes, but it's easy to clear nowadays and is really bad anyway, which means Tentacruel would really appreciate a Trapper so it's efforts to set up TSpikes aren't in vain. Plus, Tentacruel is often used a check to a few of the more difficult to check threats, but without any method of recovery bar Black Sludge it usually succumbs wayy to quickly before it does it's job to the fullest, which means it necessitates some form of Wish support to consistently check things and setup TSpikes, which at this point you'd be better off using another Mon that checks the range of threats that it does due to the amount of support it requires. The popularity of hazard stack ironically is another factor that dampers the overall effectiveness of it as it pressures it and it's Wish Passer a great deal more ensure that they are both healthy enough to check what they have to check for basic team functionality. So basically, Tentacruel needs a LOT of support to effectively be used and bring the relatively small support that it does bring to a team, this argument is sorta similar to the Mega Venusuar problem, except there's more room to argue why it should stay and the virtue of just having Toxic Spikes is also a reason.