Whimsicott - pretty balls, can't check EKiller or ExtremeSpeed Rayquaza. Doesn't set up hazards, doesn't have Sleep Powder. Might be an irritation for offensive teams, at most. D Rank.
I think its far more then "just an irritation". It gets a less accurate sleep powder in Grass Whistle but that isn't my point. I feel it should be rank C for the following reasons; it is capable of threatening everything that tries to stop its sub seed cycle thanks to its huge support movepool, and its sub seed cycle can arguable continue forever unless a Pokemon comes to stop it (in which case it has to become neutered to an extent thanks to that support movepool). It cannot go higher thanks to the fact its shutdown by Magic Bounce mons, the fact it has a major 4-moveslot syndrome (much larger then others I'll add), and the fact Toxic Spikes neuter its survivability and its hard to switch-in.
It can definitely be very effective given the right support (I'll add Ibo made an extremely successful quick stall team around this little guy iirc, and nothing else can perform this guy's niche). It pretty much defines C rank. I already listed its crippling flaws earlier. I think it should be C Rank and not D rank.
Anyways to elaborate, the Sub Seed Cycle it starts can continue forever if it can switch-in safely OR on a setup move from another mon. It can then proceed to Encore the poor mon that used a setup move should it lack priority (or just anything not named SD Rayquaza and Extremekiller Arceus), thanks to Encore. It can then do either one of these two things. It can then set up a Substitute and start its sub seed cycle the turn after, regardless of whether the pokemon locked onto decides to stay in. Assume this turn is turn 1.
If the pokemon decides to switch out turn 1, your met up with something behind a sub and priority leech seed. Not only did you just accomplish your goal and technically stop / force a setup sweeper out, you also can Leech Seed this new Pokemon and you have a free sub! Unless its a Grass type or a user of Rapid Spin... but if this Pokemon matches these qualifications we can assume that this Pokemon is either a Ferrothorn, Forretress, Arceus-Grass, Kabutops, Tentacruel or Excadrill. Ferrothorn, Forry, and Arceus-Grass can be rendered setup bait for something else with a Taunt, Kabutops and Excadrill can be crippled with Priority Stun Spore. Tentacruel is arguably the only thing that can safely switch-in and setup Toxic Spikes and Rapid Spin, but at the same time, its plausible to take advantage of this Pokemon for setup right (admittedly no Physical sweeper likes setting up infront of a Scald... but what about Calm Mind Arceus or the like?)? If this Pokemon is not a Grass-type or user of Rapid Spin you can simply spam Substitute after you've Leech Seeded, wearing this Pokemon out forcing it to use a Recovery move (should it have one) or switch out. If it has a Recovery move you can Encore + Taunt the turn after it uses Rest / Recover / w.e. If it switches out you have a substitute up and you can do everything that I listed above all over again. There are three exceptions to these cycles and that is if the pokemon out has ExtremeSpeed or a form of priority, in which case your going to either force it to not setup with Taunt, Stun Spore it, or Leech Seed it and switch. The second scenario is if Xatu / Espeon comes out and regardless of what happens in that case your basically screwed x_x. The third is U-turn breaks your sub meaning you don't have a free sub anymore when something comes in. Thankfully, the most common user of this strong enough U-turn is Genesect. If you paralyze him you've done a lot for your team AND stopped two threatening foes. That setup sweeper that tried to setup earlier, and you've crippled a revenge killer!
If the pokemon decides to stay in turn 1, it can attempt to Encore the setup move again as encore ends then Taunt the foolish Pokemon and enforce struggling or a switch, regardless of the case it can leech seed the pokemon coming in after and continue the sub seed cycle. If Encore fails it can simply spam priority Substitute to wear out the Pokemon, it will be doomed if it does not use a recovery move which can simply be Encored and Taunted unless they switch. Oh and if they do switch you have a free sub, your still going to continue your sub seed cycle scenario. The ways for escaping this cycle are the same as listed in the paragraph before this.
I apologize if this sounds like a huge rant and if this intimidated anyone x_x.