Deoxys has great offenses and a wide offensive movepool. Access to Spikes also sets it apart from most Pokemon in Ubers. Unfortunately, Deoxys does not excel at anything worthwhile compared to Deoxys-A and Deoxys-S, which both outclass it; they share Deoxys's excellent movepool while possessing more specialized stat distributions. Deoxys-A's higher offenses make it a far better choice as a fast and frail wallbreaker, while Deoxys-S's higher base Speed makes it a superior suicide lead. To stand out at all, Deoxys is left trying to make its defenses, which are barely better than Deoxys-A's, somewhat functional. If Deoxys is holding a Chilan Berry, it can act as a semi-reliable check to Extreme Killer Arceus, while holding a Kasib Berry allows it to revenge kill Marhadow. In general, however, you are better off using something else, as Deoxys contributes very little to teams on its own and is just an outright inferior choice to Deoxys-A, Deoxys-S, and more useful checks to Arceus and Marshadow.
name: Utility Check
move 1: Psycho Boost
move 2: Taunt
move 3: Spikes
move 4: Counter
item: Chilan Berry / Kasib Berry
evs: 144 HP / 252 Def / 112 Spe
Psycho Boost is Deoxys's best STAB move and does sizeable damage to most things that do not resist it or are not very specially bulky. Taunt discourages setup from the opposing team and can annoy Pokemon common on stall teams, such as Blissey. Spikes is a useful entry hazard that Deoxys can set when it has nothing better to do. Counter lets Deoxys do massive damage to boosted Arceus that attempt to use Extreme Speed. It is opted for over Low Kick, as the prevalence of Chople Berry Arceus makes Low Kick much worse; it fails to 2HKO Chople Berry Arceus, which would allow Arceus to break through Deoxys with several uses of Extreme Speed.
The HP and Defense EVs let Deoxys take any one hit from Extreme Killer Arceus. 112 Speed EVs along with a Timid nature allow Deoxys to outspeed everything up to neutral-natured base 150s, such as Rash Deoxys-A. Chilan Berry lets Deoxys survive a boosted Extreme Speed from Arceus and do excellent damage in response with Counter. Alternatively, Kasib Berry lets Deoxys revenge kill Marshadow, as it lets Deoxys survive a Spectral Thief and OHKO in return with Psycho Boost.
If you are running Chilan Berry and your opponent reveals Extreme Killer Arceus, feel free to switch Deoxys into it; if Deoxys comes in on Arceus's Swords Dance, it can then respond to Extreme Speed or Shadow Claw with Counter, doing great damage. If you are running Kasib Berry, you can switch Deoxys into Marshadow after a teammate has fainted and either force it out or OHKO it with Psycho Boost. If your opponent has neither of these Pokemon, Deoxys's resultant uselessness confines it to using whatever turns it has to set up Spikes or try to deal damage with Psycho Boost. It is total death fodder against offense when none of the Pokemon it is supposed to check are present, so play it very aggressively. Taunt can annoy some defensive teams, but Deoxys is totally walled by Mega Sableye, rendering it useless in this matchup as well.
Deoxys is relatively self-sufficient due to its extremely one-dimensional nature. Fairy-types such as Xerneas, Arceus-Fairy, and Mega Diancie can deal with the Dark-types, such as Yveltal and Mega Sableye, that wall Deoxys. Offensive entry hazard removers, such as Cloyster and Excadrill, can prevent Deoxys from being worn down, which is important because Deoxys needs to be at full health to be of any use to a team. Setup sweepers with middling bulk such as Dragon Dance Rayquaza and Mega Blaziken appreciate Deoxys's ability to deal with Extreme Killer Arceus, as this lessens the chance that they will be picked off by Arceus's powerful Extreme Speed. Other sweepers, such as Mega Lucario and Primal Groudon, can take advantage of Spikes.
Unfortunately, Deoxys cannot do anything of note to set it apart from Deoxys-A, which is a superior attacker, and Deoxys-S, which is a superior lead. Stealth Rock can be considered over Spikes, but there are better users of the move.
Checks and Counters
**Mega Sableye**: Mega Sableye is a complete stop to Deoxys, as it is immune to both of Deoxys's attacking moves and can render its attempts to use Spikes and Taunt useless with Magic Bounce.
**Dark-types**: Dark-types are immune to Psycho Boost, neutering Deoxys. Yveltal, Arceus-Dark, and Alolan Muk can easily switch into Deoxys and OHKO it with their Dark-type STAB moves.
**Ghost-types**: Giratina-O can take a Psycho Boost and OHKO Deoxys with Hex. Marshadow can revenge kill Deoxys with Shadow Sneak, but Life Orb variants will lose if they try to take on Kasib Berry variants one-on-one. Lunala takes very little damage from Psycho Boost and is immune to Counter, letting it use Deoxys as setup fodder.
**Special Attackers**: Deoxys's lack of Special Attack investment and reliance on Counter mean that several special attackers, such as Primal Kyogre, Xerneas, and Palkia, can switch into a Psycho Boost and OHKO Deoxys with any one of their offensive moves. All support Arceus formes viable in Ubers can perform a similar feat due to their great bulk and powerful Judgment.
**Faster Pokemon**: Anything faster than Deoxys, such as Naive Deoxys-A, Mega Mewtwo Y, Jolly Pheromosa, and Choice Scarf Xerneas, can easily revenge kill Deoxys. Mega Mewtwo Y doesn't even mind switching into Psycho Boost.