Rankurusu should be banned.
And here's why:
Many of you will remember a thread by DougjustDoug. It was entitled, "Charateristics of a Desirable Pokemon Metagame", and in it, people attempted to answer the question of what kind of metagame we wanted to create. The second point on that list is as follows:
The metagame should have the widest possible variety of playing options and strategies that are viable and competitive for knowledgeable players.
Now let me explain how this relates to the banning of Rankurusu. As most of you will no doubt know, Rankurusu is available with the ability Magic Guard. This ability makes it immune to all forms of passive damage: it must be taken out by direct attacks. But how does this relate to the banning of Rankurusu?
One of the main playstyles in copetitive Pokemon is Stall. Heavy Stall, the main subfacet of this, is the intention to eliminate all of your opponents Pokemon using various forms of passive damage. Whether damaging weather, Entry Hazards or Status, eventually, the enemy team succumbs to the conditions and is defeated. But Rankurusu makes this entire playstyle totally unviable. Let's start with the obvious. Magic Guard means it is unkillable using passive damage. However, there is another Pokemon with access to Magic Guard: Clefable. Surely then, Clefable should also be banned?
The problem with this mentality is that although Clefable is immune to passive damage, there is little else it can try and do. Disregarding Magic Guard, it is almost completely outclassed by Blissey and Chansey as a defensive Pokemon, and as an Offensive Pokemon, there are many better options. Compare thier base Defenses:
As we can see, Rankurusu is clearly superior in Defense, and slightly superior in Special Defense. Now lets see how they individually deal with commonly seen 5th generation Pokemon:
To start with, we'll look at Roobushin. The standard Bulk Up set, which is becoming very common in the 5th generation metgame due to it's ability to counter many top threats.
+2 0 Atk. Roobushin Payback Vs. 252/252 Rankurusu - (124-146)+1 0 Satk Rankurusu Psychic in Return - (330-390)
+2 0 Atk. Roobushin Drain Punch Vs. 252/252 Clefable - (380-450)
Clefable is generally unable to hit Roobushin in return.
As we can see from these calculations, even a +2 Super Effective attack coming off a powerful pokemon barely scratches Rankurusu, while it deals heavy damage in return. I have chosen Roobushin since it is sometimes seen on stall teams as a counter and utility check.
Another offensive pokemon seen on stall teams is Tyranitar. I'll assume a fairly bulky spread with 80 Atk Ev's - this is still Stall after all.
Tyranitar Crunch Vs. 252/252 Rankurusu - (224-266)
+1 Focus Blast in return - (420-496)
Tyranitar Stone Edge Vs. Clefable - (144-169)
As we can see, Rankurusu takes heavy damage from the Crunch but is not Ko'd, and rips Tyranitar apart in return. While Stone Edge does not do huge damage to Clefable, all it is really able to do in return is Thunder Wave, which can be easily dealt with by a cleric.
A final Point I would like to make is that on it's own, Clefable cannot beat stall. It gets Softboiled, but it's only real boosting move is Belly Drum, which is very risky and forces it to run a Physical set, meaning Desukan can easily remove Magic Guard and allow Clefable to be killed by passive damage. On the other hand, once stall is unable to force Rankurusu out with Roar, it will often sweep through the team, forcing the stall player to PP stall it if he or she wants to have any hope of beating it. Due to the decent amount of PP Rankurusu usually finds available, this is very tedious and difficult. Clefable can also be crippled by a Burn, whereas the only status which worries the Psychic Type is sleep, and a Heavy Stall Team has no real way to capitalise on the free turns it gets in this manner.
So to conclude, (or just as a tl;dr point) I believe Rankurusu should be Banned because it's ability allows it to remove Heavy Stall as a viable playstyle, thereby infracting the second point of DougjustDoug's "Characteristics of a Desirable Pokemon Metagame".
From Hell's Heart I Stab at Thee