Ditto, branded as a useless whore for the last four generations, returns as a sassy pimp in all its pink and purple glory in an attempt to dominate the Pokemon world. Its new Dream World ability, Imposter, allows Ditto to turn into the opponent bar item choice and HP without wasting a turn. With a Choice Scarf, Ditto will almost always be faster than the opponent, and functions as the best revenge killer there ever was. If that doesn't sound perfect enough, Ditto is also an all-star scout—the moment it transforms into the opponent, Ditto can reveal its opponent's entire moveset. Ditto can also switch into a myriad of Pokemon immune to their own primary STAB, or steal opposing utility moves ranging from Aromatherapy to Whirlwind.
However, Ditto is far from perfect. How so, you ask? Ironically, Ditto is only as strong as its opponent, and this flaw allows Ditto to be at its worst in the weakest metagame. Most importantly, NU mainly lacks Speed-boosting setup Pokemon that Ditto can take advantage of. Calm Mind is perhaps the most common boosting move in NU, and Ditto can do little to prevent Calm Mind sweepers such as Musharna from rampaging through your team. Still, Ditto can easily revenge kill prominent threats such as Gorebyss, and acts as a reliable "panic button" of sorts. Most importantly, Ditto is an incredibly fun Pokemon to use, and can provide you—and your Pokemon—with some good old-fashioned fun.
This is no doubt Ditto's best set. Imposter functions as a free Transform, immediately copying all base stats bar HP, all stat changes, and the target's exact moveset. With a Choice Scarf, Ditto turns into a monstrous revenge killer, as it can outspeed the opponent no matter what. Of course, Ditto will Speed tie with opposing Choice Scarf users, but it can revenge kill set-up sweepers such as Shell Smash Gorebyss and Belly Drum Linoone, as well as weather sweepers such as Growth Victreebel. Additionally, Ditto can take advantage of an opponent's immunities in order to switch into threats such as Choice Scarf Zebstrika. In a similar fashion, Ditto can also switch into Sap Sipper Sawsbuck, though not without considerable risk. Keep in mind that Ditto is so frail thanks to its low base HP that it is rather difficult to bring it in on anything other than weak resisted hits such as Choice Scarf Sawk's Stone Edge. Take note that if you need Ditto for revenge killing duties, it might be better to avoid taking such risks. Furthermore, Ditto can function as a scout as it can copy each of the opponent's moves, revealing any surprise coverage options. If necessary, Ditto can even act as a lead, and steal the ability to lay entry hazards from opposing Cacturne or Garbodor. Ditto plays as a revenge killer, scout, and late-game cleaner all in one; however, as Ditto lacks any surprise moves or spreads, remember that Ditto will be only as good as how you play it.Team Options & Additional Comments >>>
Ditto has few other options. It can choose a different Hidden Power type, but very few Pokemon in NU use Hidden Power as a main attacking option. Moreover, those that do, such as Duosion, generally beat Ditto anyway. Ditto can use something other than Choice Scarf, but Ditto will always face Speed ties without it, and loses its largest niche as a reliable revenge killer. Really, unless you have an unsavory mind or a fifth-grader's imagination, there's very little else Ditto can do.
Checks and Counters
It is hard, if not impossible, to name a single Ditto counter in a traditional sense; Ditto is not your average Pokemon, after all. The most natural thing to do would be to try and check whatever Ditto is at the moment. In fact, you know all of its moves, so you can rest easy that there are no surprises waiting for you. Additionally, Ditto will almost always be holding a Choice Scarf, so if you can switch in a Pokemon that resists its chosen attack, Ditto will certainly lose its momentum. Furthermore, Ditto lacks a Life Orb or Choice Band, which further increases the number of Pokemon that can take an attack and hit back. Unless those Pokemon are worn down first, Ditto will have a hard time being effective.
Nonetheless, there are some things that Ditto will nearly always have trouble with. Substitute, which prevents Imposter from taking effect, makes Ditto essentially useless; Ditto will hence find it impossible to revenge kill setup sweepers such as Substitute + Shell Smash Gorebyss and Substitute Klang. In addition, Ditto takes damage from all entry hazards, and will be affected by Toxic Spikes even if it turns into a Poison- or Steel-type.
By far the best way to check Ditto, though, is a little thought. When building your team, make sure that you can take advantage of opposing Choice Scarf Pokemon. If you have a Pokemon that, when given +1 Speed, can beat your whole team, do not allow Ditto to copy that Pokemon. When you see Ditto in Team Preview, hold on to your Swords Dance Sawsbuck and Shell Smash Gorebyss, unless you are confident that you can revenge kill or somehow neuter the opposing Ditto. Finally, be careful not to give Ditto entry hazards or sleep-inducing moves, as they tend to be an incredible nuisance to face.