I doubt Rankurusu will be OU. It has terrible STAB, is Pursuit weak, and has very low Speed. Trick Room still needs a turn to be set up, so Trick Room teams will suffer from the same problems this generation as the last. Stall is looking less and less viable due to unblockable Rapid Spin and numerous hard-hitting Pokemon, so Rankurusu will be a very niche Pokemon when used as a wallbreaker. I can see Rankurusu being a top UU contender, though.
That was posted in September. Lawl at Smogon, since now everyone's saying he might end up in ubers.
Saying Reun (I'm ok with the short version, but his whole name is lame. Lame in Japanese too, though) can completely demolish Stall teams and telling people to offer up an example of what you can put into a stall team to deal with him isn't the point of the metagame, guys. Note that when 4th Gen's metagame got going it marked a shift
away from stall. Stuff like Infernape allowed people to conquer the mighty SkarmBliss.
Pretty much everyone has already talked about the power creep Gamefreak has been doing. This is just continuing the trend. The fact that 5th Gen metagame is currently based around lots of switching followed by revenging back and forth for a bunch of turns is the hand we've been dealt. So instead of trying to adjust the stock of Pokemon/moves/whatever to "balance" the metagame to whatever people see as the ideal, it's easier to stop clinging to stuff like stall and work out new strategies.
This gen got a number of bulky offense Pokemon, Reun included. Along with something like Shibirudon (awful speed, but decent bulk, great movepool, and no weaknesses) you can work on a solid TR team. Moves that people have previously used only as gimmicky sets could also turn out to be useful (Gravity means you can fire off more accurate Thunder/Blizzard, which offers a pretty significant power boost to a number of Pokemon who would otherwise use lesser forms of Boltbeam or whatever. I haven't really looked much at Wonder Room or... was it Magic Room? But those are things that could, after one turn of setup, screw up your opponents pretty thoroughly).
So yeah. Reun shouldn't be shipped up to ubers. And I feel the same about Swift Swim and all that business too.