There are no tiers established yet. But if you want to choose Pokemon, I like to use the "500 base stat" rule. Usually anything above 500 total base stat will go in OU, below about 480 is usually safely in UU. In the middle could be either.
Darmanitan for example will almost certainly be OU, whereas Bisharp may be UU (because of low speed). There's a small possiblity that Chandlure and Volcarona will be Uber but probably not.
You can look at stats here: http://pokemondb.net/pokedex/all