Ok, going to cover Suicune and Mega-Blastoise. Would also cover Krookodile, but I simply don't have experience with it to form a viable argument one way or the other:
Suicune: I think A+ is pretty good. If you don't have a way to beat Suicune, you're fucked. However, it has a fairly major issue; Slowbro. Slowbro can not only give it competition as a bulky Water-type in general, but also as a win-con for stall teams. CM Slowbro is able to double up by not only handling some top physical attackers like Mienshao, Darmanitan, and Victini, but it can also utilize Regenerator to slowly get itself back in shape to sweep late-game. As for Suicune, it has to use Rest to get back in shape for sweeping late-game. Not only that, but a quick CB Bolt Strike, Reckless HJK, or repeated U-turns will have much more of an impact on Suicune who can't passively regain HP via Regenerator. Suicune is definitely something you need to be ready for, but you have to cover for more things if you use it than you would if you used Slowbro. The main advantage Suicune has is the fact it doesn't give two fucks about poison, but considering that Slowbro has some pretty solid synergy with Florges and Umbreon, two of the best clerics in the tier, this usually isn't a major issue; just a mild inconvenience.
Mega-Blastoise: This is a hard one for me. On the one hand, it's arguably the best spinner in the tier. Mega Launcher Dark Pulse screws every Ghost-type in the tier. It has a great Special Attack stat and solid coverage to be able to break down some common defensive Pokemon. However, it has two major drawbacks that make it A rank.
The first being its Speed stat. Sure, it can beat most Ghost-types, but with a lackluster Speed stat and the inability to have any form of healing, even Leftovers, makes it super easy to wear down into KO range of many top attackers. Just a couple of examples: LO Regen Mienshao only needs 26% prior damage to ensure a KO with HJK; an easy feat when factoring in Stealth Rock and a minor hit from most Ghost-types. Choice Scarf Victini needs ~35% prior damage to finish the job with Bolt Strike. Band Victini? a measly 3%. The standard Life Orb Honchkrow needs just 25% prior damage to KO with Brave Bird. Now, you're probably thinking "Eon, these threats aren't Ghost-type. Mega Blaster can spin on them and fulfill it's role. Um, no. So long as there's a Ghost-type of virtually any kind paired with these threats, Mega Blaster is going to be outsped and in KO range of these threats after a single encounter with most Ghost-types. And it should be taking hazard damage because it's job is to spin them away.
Now, the other major issue. Toxic Spikes. It is incredibly vulnerable to Toxic Spikes, and we don't have a lot of good grounded Poison-types in the tier right now. Roserade, Nidoking, and Nidoqueen are about the only three grounded Poison-types. Two of them can viably run Toxic Spikes as well, which makes matters worse as they are at least decently common. Mega Blaster has no recovery of any kind unless it's a defensive RestTalk set, which kinda sucks considering RestTalk and Spin would be near necessities. So, Mega Blaster would go from just losing 12.5% when it comes in to losing 12% a turn. This is just going to lower it into more KO ranges that much faster, making it even harder to spin. Now, is it bad? No. It's about the best bulky spinner we have as it does shit on most Ghost-types easily and is at least able to outpace most walls. However, it can be a bit suboptimal without recovery and the fact that many offensive teams can simply use a Ghost-type to prevent a spin and just get the appropriate faster threat in to pressure it out to prevent the spin ultimately. Compare this with Starmie who can still beat a fair few Ghost-types with the brute force of Hydro Pump and have the Speed to stay in front of a fair few offensive Pokemon (Shao, Darm, Tini, Celebi, etc.) to spin. All of this makes Mega-Blastoise A rank imo. Solid spinner, but it does have some shortcomings that can easily prevent it from doing its job in some situations.