I've been playing around with Gen 6 Custom matches in Showdown, and the best Pokemon (excluding Ubers) are Aegislash and Hawlucha. Most 6th gen Pokemon seem bad at first, but at a second glance, are much better than they seem.
Barbaracle is a nightmare to deal with because he's arguably a better shell smasher than Cloyster, Noivern's coverage is god-tier, Hawlucha has amazing moves to back up it's STAB, Clawitzer has a perfect ability for his moveset, Diggersby is Azumarill 2: Electric Boogaloo, Vivillion gets CompoundEyes Hurricane, Sleep Powder, and Quiver Dance, Helioisk has good abilities and good coverage, Klefki gets Priority Spikes and Thunder Wave, and the starters are all good in their own way. They'll all do great in RU-OU.
