Deoxys (Speed Forme)

In the main series of games, Deoxys Speed Forme isn't the strongest Pokemon by itself, but its insanely high speed and great moveset made it an extremely strong support-Pokemon that could easily set the stage for a victory if played right. Sadly, here in Pokemon Go, we don't really have support roles, and speed doesn't translate into this system extremely well, making Deoxys-Speed the single most awkward Deoxys form out there.

Deoxys-Attack has insane raw power that makes it into the single glassiest glass cannon out there, Deoxys-Defense has good power and staying power that makes it a king in PVP Great League, and even normal Deoxys has a niche as a potential electric attacker, but poor Deoxys-Speed sits with nothing to call its own. Unless something changes, Deoxys-Speed joins the ranks of other legendaries that did not translate well into the environment of Pokemon Go such as Registeel and Suicune as dex-fillers, and sadly nothing more.  

Best PvE Offensive Moveset

• Zen Headbutt is Deoxys-Speed's strongest quick-move.
• Charge Beam has a small niche if you want to use Deoxys-Speed as an electric attacker, though it's painfully weak in that role.
• Thunderbolt is Deoxys-Speed's best charged move, having multiple bars and out-damaging Psycho Boost even without STAB.
• Zap Cannon is entirely inferior to Thunderbolt and is not viable.
• Psycho Boost can be used with Zen Headbutt if you're desperate for a psychic-type attacker, but even budget options such as Espeon beat it easily in this role.
• Swift has no use on Deoxys-Speed.

Despite lacking STAB, Charge Beam is more preferable due to its significantly higher energy generation. Zen Headbutt is underwhelming even with STAB. 

Psycho Boost is the primary charge move for Deoxys Speed Forme due to its low energy cost and high DPE rating backed by STAB. Thunderbolt is a good move in PvP and offers some form of coverage against various common Pokemon. Swift performs very poorly in PvP.