Celebi has a known history of metagame-related pressure; with every new generation, its niche in OU becomes ever harder to maintain because of the need to cover new threats. With the transition to the fifth generation, classic rivals such as Scizor and Tyranitar are more popular than ever, while the emergence of newer threats, especially Latios, Latias, Hydreigon, and Chandelure, threaten Celebi's existence in OU. This problem isn't helped by the fact that Celebi didn't receive anything significantly new in the transition, forcing it to resort to the classics.
Despite these unnerving flaws, Celebi only actually requires adaptation to function in the tier it has comfortably inhabited for three generations. While Celebi's typing brings it many crippling weaknesses, it also grants Celebi resistances to six common attacking types. This, in conjunction with its 100 / 100 / 100 defenses, allows Celebi to sponge many attacks, even some super effective ones, and support its team with ease. Celebi isn't bad offensively either, having access to an adequate offensive movepool as well as the ever-useful Nasty Plot to destroy the most common Pokemon in OU. With the correct support, Celebi will always be a valuable addition to a team, capable of easily paving the way for a teammate.
<p>As most legendary Pokemon, Celebi was blessed with great Base Stats all around. It also has a quite good and synergistic typing, an awesome ability that lets it act as a status absorber, and a phenomenal movepool. These traits allow Celebi to play many roles, depending to your teams needs. It can be a sturdy special wall that handles many prominent threats, such as Keldeo, Thundurus-T, Sheer Force Landorus, and Breloom, with a large repertoire of utility moves, consisting of Stealth Rock, U-turn, Perish Song, Thunder-Wave, Heal Bell and Baton Pass. Or it can take the offensive route and abuse its good coverage, bulk and resistances to set up with Nasty Plot, making it quite hard to wall. It can even use a choice scarf set to revenge kill troublesome threats, a baton pass set to assist your sweepers, or a tank set to combine the offensive and defensive roles.</p>
<p>However the list of its flaws is as big as the list of its advantages. His typing leaves it with a whopping seven weaknesses, one of them being a quadruple weakness to Bug moves. This means that it is prone to both Pursuit trapping, and to U-turn, which is never a good thing with Genesect lurking in the corner. As most Grass-types, it struggles quite a bit against sun teams, and finally is hurt by the prevalence of Tornadus-T. But don't let those flaws disappoint you. With the right teammates Celebi can really shine, and prevent those pesky rain teams that are everywhere from overwhelming you.</p>
Here is the Celebi Overview. I have mentioned that Celebi struggles with Genesect and Tornadus-T around, but i don't think that the overview should be particularly negative, as Celebi is still a pretty good poke that has a lot of good qualities, such as being the best Keldeo counter in the tier, and checking Thundurus-T, SubCM Jirachi, and rain teams in general. Not to mention it is a fantastic pivot with exceptional utility moves.
<p>Although Rotom-W didn't gain any improvements in BW2, it has managed to stay in the spotlight due to its ability to handle many new threats such as Keldeo, Genesect, Tornadus-T, and rain teams in general, which are on the rampage, in addition to the Pokemon it used to check. Its awesome typing, useful ability, and great dual STABs make it one of the few Electric-types that can beat Tyranitar and check Ground types, while also allowing it to fit on any team with little worries; it performs especially well in team archetypes revolving around the VoltTurn strategy, mainly offensive ones. Alongside Scizor, Genesect, and Tornadus-T, it can run circles around whole teams with little effort, making it a very annoying Pokemon to face. Rotom-W may not have a lot of options, but it does its job well, and has carved its own niche as a premier VoltTurn partner.</p>