Volbeat deserves to be in C tier, which is the same as Whimsicott.
Prankster Volbeat has garnered his(?) fame thanks to priority Rain Dance or Sunny Day, but Sunny Day is generally performed better by Whimsicott. Basically, Volbeat is the 'Politoad' of RU, in that he can set up rain 80% of the time. Additionally, the ability to pass Tail Glow, and Substitutes can make some special sweepers really fearsome. Encore is the icing on the cake to stop setup sweepers in their tracks.
The downside is that Volbeat is Stealth Rock weak, so it won't be able to switch in too many times, also it isn't the bulkiest Pokemon either, but its bulk still serves its purpose. Also, Volbeat is utterly destroyed by Taunt, but it will usually pass away before that can happen!
Volbeat for C tier.
Edit:
B rank said:
Reserved for Pokemon who are great in the RU metagame. These Pokemon have more notable flaws than of those above it that affects how they function in the tier. Their positive traits still outshine their negatives, but they require a bit more team support to bring out their full potential.
I agree with this.
The positive traits that Crustle brings is notably Spikes and secondly Stealth Rock. Good hazard setters a hard to find and Crustle is a good one, thanks to its massive base Defense and alright HP. Additionally, two weaknesses means that Crustle has an easier time switching in and setting up more hazards, albeit those weaknesses are a bit common.
Now those flaws, Crustle's weakness are definitely up there, as Water- and Rock-type attacks mean that Crustle will often have to switch out. Also, a paltry 75 Special Defense and 45 Speed isn't winning Crustle any favors either. Since Crustle is mainly a hazard setter, a spinblocker is implied, unless its a fast paced team, but then why not use like Accelgor? A typical Ghost-type would be Spritomb and Rotom, which are both in A rank, so having them isn't a
bad thing, so that support isn't too much of a hassle. The biggest is probably the Speed and general weak Special Defense, but that's why there are partners!
Crustle for B rank!
Edit v2:
I don't think C rank does justice to our little rose-thing.
Roselia is a specially defensive behemoth that can also lay down Spikes, which makes it even more flavorful!
The perks to Roselia is quite long in that she can tank hits from a lot of Pokemon in ranks above her. Non-SD Sceptile, Lanturn, Lilligant, Poliwrath, Rotom(s), Tangrowth, and those are only Pokemon that Roselia can easily handle. Roselia can also lay down Spikes to punish the opponent further. If needed she can take statuses in a pinch thanks to Natural Cure, and can constantly heal with either Rest or Synthesis so she ain't going away. The perks that Roselia brings is that she can tank a host of special attacks, set up Spikes, and can be around a while.
Roselia shouldn't be any further than B rank, though. Fire-types are everywhere, Entei and Moltres, and their STAB will run right through Roselia. Also, a puny base 45 Defense doesn't do her any favors in taking physical attacks (kinda like Cryogonal), so Escavalier and Durant can run her over.
Another downside is that Roselia should be paired with a spinblocker, probably Spritomb or Rotom, but because they are both excellent Pokemon that isn't too big of a deal, although that could place a restraint on teambuilding.
Roselia for B rank!