I think Deoxys Attack. Since it's already TOO frail so even lowering it's defense more would be nothing for it. It's also better for Deoxys-A to make it faster, because it's very frail, and if it's lower then another Pokémon's speed then.... You know what's gonna happen. Normal form would be OK too, but still, having lower defense then 50 (or 50) with a lower thing in your best stat is worse then having a Pokémon with 20 defense with having then +30 in their best stat. Deoxys Speed could be an OK option too, since you are supposed to be the fastest, but since Deoxys Speed is already the fastest Pokémon it isn't the best thing :/ If it's in competetive no one is gonna use a Ninjask, so even with Ninjask outspeeding a Deoxys Speed with a Choice Scarf, no one would use it, because it's in PU. So my point keeps at Deoxys Attack, it has great Attack Moves, like Zen Headbutt, with a 20% chance to flinch, so if some Pokémon still survived your hits, then there is a chance that the opposing Pokémon will flinch. Now, I am a bit making a moveset for it, but you get it. I am not the best competetive wisely so you should test it out by yourself. (I am thrash competetive wisely, so I don't answer :P)