Deoxys-S personally is my favourite form for one reason: It can revenge kill without being locked into a move. I know it's already been discussed in earlier forums, so I won't go into deep detail, but here's the set I use as of now (Two EV spreads, lol)
Deoxys-Speed @ Life Orb
Ability: Pressure
EVs: 252 Spe / 120 SAtk / 136 Atk
OR 212 Spe / 160 SAtk / 136 Atk
Rash Nature
- Psycho Boost
- Knock Off
- Ice Beam
- Superpower
So lets go into the EVs as there are two spreads. The first one is my personal preferred one, 252 in Speed for outspeeding just about everything that isn't scarfed in the entire game; Ubers or Not. We put 136 in Atk for a guaranteed OHKO with Stealth Rocks down on Mega TTar, and to make it more perfect, 120 SAtk allows an OHKO on Physical Bulky, or All Out Attacking Mega Venusaur. The Second spread is very similar, we have 212 to outspeed +1 Adamant Charizard (Trust me this has saved my life) Atk is the same, and rest poured into SAtk so Psycho Boost hits like a bomb. Now onto the set, Rash Nature is there as Deo-S is a mixed attacker, Psycho Boost is just... don't even go about out powerful it is (Only if Deoxys got Contrary). Knock Off is a move I haven't seen that often, it's a VERY good move and is more reliable then Ice Beam to take out the Lati Twins, or other opposing Psychics (Not very good for Aegi as you get a 3HKO unless you get a crit). Ice Beam and Superpower is our coverage.
Let me note that this set (or the moveset at least) was from Pokeaim, I just put it at what I thought were the best EVs. Honestly, I think the biggest problem with Deoxys-S is that it can't use all these natures. For example, It get's outsped quite easily from a lot of scarfers unless using a Speed + Nature, but that leaves Deoxys with okay, but pretty bad attacking stats, sometimes leaving it just too weak to sometimes even 2HKO things! By the way, what is your preferred Speed EV on Deoxys-S? And if you read everything here then you my friend are a gentlemen.
Edit: Here's a replay of Deoxys-S cleaning a battle the last three turns:
http://replay.pokemonshowdown.com/ou-104606154