Tier
Template:Protected Some competitive fans divide Pokémon into five different tiers based on competitive ability. There are currently six tiers with various fansites individually deciding the Pokémon assumed to be in those tiers, most notably, Smogon. The Pokémon tier list has received much criticism, because there is no "official" tier list for Pokémon.
Most battles are conducted using every tier except Uber. However, it is common for some battles to maintain use in one tier only. Usually Under-Used only, or Never-Used only.
Uber
The Uber tier is often characterized by legendary Pokémon. The Uber tier often exists as a banlist for standard Wi-Fi battles. Pokémon in the Uber tier are deemed too overpowered to be used fairly among other Pokémon.
While most Pokémon in the Uber tier are there for their stats, Wobbuffet is included primarily because of its Shadow Tag ability, and "uncounterable" movepool of Counter and Mirror Coat. Since the principle behind countering a Pokémon in battle is to switch out to a Pokémon that is better able to cope with the opponent, and Shadow Tag prevents switching altogether. Wobbuffet's pre-evolution, Wynaut, is often banned for similar reasons.
Garchomp is also a non-legendary that some fans consider for the list due to its high stats, movepool, and low amount of weaknesses.
Over-Used
Over-used, commonly abbreviated as "OU", refers to Pokémon that are most commonly seen in standard play. It is a misconception that all OU Pokémon are the most powerful in the game. OU is a term based on usage; however, there is a distinct correlation between usage and usefulness in the system.
There are exceptions to this such as Magneton in Generation III, where it was considered an OU Pokémon merely because it was used primarily as a counter for Skarmory; a Pokémon commonly seen due to its high defense, low weaknesses, and stall-based movepool. The introduction of Magnezone, an evolution of Magneton, solidified its place in OU, due to increased stats, and a broader movepool.
Examples of common OU Pokémon are the aforementioned Skarmory and Magnezone as well as Blissey and and all of the pseudo-legendaries like Tyranitar.
Borderline
Pokémon in Borderline, commonly referred to as "BL", are considered by some to be too strong for Under-Used, but may be found in some OU battles. They are classified as BL because they simply do not have the capacity to compete in solid OU consistently due mostly to stats, or poor movepools.
Examples of BL Pokémon are Jynx, Smeargle, or Ludicolo.
Under-Used
Under-Used, commonly referred to as "UU", designates Pokémon whose use is limited as they are not strong enough for general BL, or above, competition. Usually due to poor stats, poor movepools, or easily exploitable weaknesses. UU Pokémon are recommended by some solely for UU type battles.
Examples of UU include Butterfree, Rapidash, or Toxicroak. The latter is an excellent example as it has a good movepool, but a 4x weakness to one of the most common types in OU battles, Psychic.
Never-Used
As the name implies, Never-Used, commonly referred to as "NU", is for Pokémon that have problems competing even in UU due to dismal stats and/or movepools. This is where most novelty Pokémon reside in tiers as well as Pokémon found early in a player's journey. They are recommended by some only for use only designated NU battles. However, most NU Pokémon are very common among contest coordinators.
The most notable examples are the aforementioned novelty Pokémon, like Pachirisu or Chatot, early, easily obtained Pokémon, like Beautifly or Ledian, but can also include moderately powerful Pokémon like Magcargo or Exploud.
As of now, some websites, most notably, Smogon, have completely gotten rid of the NU tier, and instead, moved all of the Pokémon associated with it, into the UU tier. This is most likely due to the Generation IV move split allowing for any type to have moves that are physical, or special; thusly clearing up any stat inconsistencies.
Tier variations
Some Pokémon are not listed in tiers but are sometimes used in competitive play because they have a change during evolution that changed their disposition enough to allow them to stay competitive, even in an unevolved state. There are a few exceptions, however, such as Porygon2, which is due to its evolution, Porygon-Z having lower defense, and Light Ball Pikachu.
Another example would be Marowak when used in battles when items are not allowed. Marowak with a Thick Club doubles its base attack to 160, easily putting it in OU. However, without the club, its base attack remains 80, putting it in the UU, or NU in some cases, tier.
