Intro
Introduction to Pokémon Tiers
First things first. Before we have a list of the Pokémon present in each tier, we should first define the tiers themselves. What is the OU tier? BL? UU? How are the respective Pokémon for each tier determined?
The tier list is separated into three actual playing modes (Overused (OU), Underused (UU), and Neverused (NU)).
In addition to those three, two more categories, Ubers and Borderline (BL), exist as "meta-tiers" to keep the playing field in OU and UU level and fair, in addition to maintaining the dual ranking nature of the tier lists: a combination of power and popularity, or usage.
Therefore, for the sake of defining tiers, Pokémon in the OU and BL tiers are considered equal in terms of power. BL exists as a tier which serves to define Pokémon that are not literally "overused," but are still too powerful to be allowed into the UU tier. Ubers exists for a similar purpose: to maintain the balance of the OU tier by removing Pokémon deemed too powerful for it through analysis and competitive play.
Each playing "mode" inherently includes lower tiers; therefore, arguments like "<Pokémon> should not be UU, because I want to use him in OU" are completely invalid because there is absolutely nothing stopping you from using UU-tiered Pokémon in OU play.
Tier Definitions:
Ubers - Pokémon too powerful for OU; a catch-all playing field. Anything goes as far as Pokémon allowed.
OU - Pokémon who are used frequently in battle. Since our focus is on battling COMPETITIVELY, one can safely assume that Pokémon used often in battle are stronger than most other Pokémon because the object of competitive play is ultimately victory in battle, which is achieved by using a "powerful" team.
USED OFTEN, STRONG
BL - These Pokémon are not seen as often as those in the OU tier, but are considered too powerful for UU play. These Pokémon ARE NOT considered any weaker than Pokémon in the OU tier.
USED OCCASIONALLY, STRONG
The need for a BL tier may at first glance seem questionable. However, it is important to represent usage as well as power in the tiers. Those Pokémon in the OU tier serve as a threat list of sorts; they tell you what Pokémon to keep in mind and what one should aim to counter when building a team. This is highly useful for veteran players and invaluable knowledge for people new to competitive battling.
UU - Pokémon not used often, that are relatively weak.
USED RARELY, WEAK
NU - Pokemon considered essentially unusable.
NEVER USED, VERY WEAK
