Lol @ Deo-S still being S rank.
...
can somebody count how many times I defend Deoxys-S?
Deoxys-Speed is far from an ideal sweeper.
So a Pokemon that can outspeed Choice Scarf Genesect, 2HKO threats with Super-Effective damage (against specific threats, of course, but the threat is undeniable), and has JUST enough bulk to survive non-Super-Effective Priority some of the time is far from an ideal sweeper?
At best, it functions as a lead/revenge killer-good at the former job, mediocre at the latter. The only thing I could see is "performing multiple roles effectively", and 2 roles that it always takes is hardly "multiple roles".
Dual Screens and TrickScarf. It can also perform an odd Damp Rock Rain Dance set if you don't want to clog Tornadus with that move. Those sets on top of Hazards, Offense, and even a mix of them in some ways, make Deoxys-S unpredictible in many ways.
While it can have diverse attacks, it needs to abuse the most powerful, stat reducing ones to be effective, meaning it will never be able to truly sweep a team.
Fire Punch? Rock Slide? Ice Beam? Those have downsides? Sure Superpower and Psycho Boost, but you can switch between them THEN switch out. I also love how you're mostly focusing on Deoxys-S as an offensive Pokemon, but I completely think that its best set is support, whether it be Screens or Hazards.
I would suggest it being more along the lines of B rank, as it can't sweep significant portions of the metagame, but fulfills the niche of offensive lead extremely well. A rank would be a possibility due to its great job at supporting, but its team support is limited to hazards.
Team support limited to hazards? Wait a minute, let me look...
Taunt
Magic Coat
Trick
The Fastest Knock Off in the Game
Magic Coat
Okay, I think the alien's movepool speaks for itself.
There is no single Pokemon that can outclass Deoxys-S in every way. You can argue Ferrothorn for hazards, but it can't attack (not well, at least) or bounce hazards back. Espeon and Xatu can bounce back things, but can't really do much as far as attacking (not well, again) or setting their own hazards. Espeon and Klefki can set screens, but can't be well on the offense. Some Pokemon take the Revenge Killer role well, but don't typically do well with support. Deoxys-S can attack and support and you won't know until it's too late. I run a Spikes/Taunt/Magic Coat/Ice Beam set and the Ice Beam throws everyone off. It gets Rotom-W into the field to tank the attack and I can Magic Coat a Wisp or Toxic back to it. You won't know every move...it's impossible. When it comes out doesn't help, the item typically doesn't help, and while you know it's there, you NEVER know what it will do to you. Deoxys-S has no Pokemon that competes for all of its roles, giving you options and mind games to abuse. Deoxys-S-Rank for...well, you know (you know, it's going to be Suspect Tested anyways, so even if you try to make it sound bad, it's not dropping).