So I also got requirements.
https://gyazo.com/d058ce5911dafb018c3d1074007a0441
....It's been a while, huh? Everything kind of... just caught up to me. And I still have stuff to deal with. But enough about me. The important part here is this space alien.
After much consideration, I wish to vote to
ban Deoxys-Speed.
The reason isn't actually in the Pidgeotite set, though having another set that can efficiently break a ton of things that aren't -atespeeders or very bulky Steel or Dark types really helps. (And Zap Cannon gets another spreadsheet as to its goodness, we'll return to that at another point...) That's a good set, but it's not what I wish to talk about.
It's about Gyaradosite. The type of lead that Deoxys-Speed is makes the whole beginning of matches with it involved pretty unhealthy. In at least 90% of matches, if not more, Deoxys is getting at least one hazard off. If mispredictions are involved, that number becomes 2. This coming from the fastest thing in the game, and having access to a powerful STAB move in Psycho Boost to rebuke weaknesses such as Keldeo and Terrakion if they happen to get cheeky also doesn't help its case.
..But as terrifying as the threat of a base 160 speed lead with Mold Breaker, Taunt, and hazards is... this undersells what Deoxys really is. ...
A mind-gaming monster.
The most damage done by Deoxys-Speed is not actually done in the match itself, but
before the first turn even happens. Confused? You might be. But I'll help make it clear to you. Gyaradosite and Pidgeotite Deoxys are very different kinds of threats (and this is without getting into the niche sets such as the physical or mixed stones) that require different answers to properly address. Many people are so used to Deoxys-Speed being a lead that they will instinctively put a counter to the Gyaradosite set if they see it in the preview. If they lead with something else (or the Pidgeotite set)... that's a serious leg down on an offensive team already. It's not inherently broken, persay (which is part of why it stayed in both gen 6 and 7 for so long), but is incredibly draining on both the health of Mix and Mega, and the versatility of leads.
It isn't just Toxapex and a select handful that can hold Gyaradosite or Ampharosite effectively. There are tons of effective leads with their own twist on the standard fare. Heck, Sivally has its own niche in Parting Shot.
Hopefully this, along with the other things that you have seen, will allow you to go out and say you are properly informed a little more about Mix and Mega's inner workings... they're pretty special. And hopefully you all have an opinion on that, as well...