Its Toondude, back again with another answer over a response to a popular legendary.
Last time, I had responded over a solution to Thundurus, who hopefully wasn't a failure for the time being, but I decided to do something different...
So, I present to you:
Gen 1 Mewtwo!
- Substitute / Ice Beam / Blizzard / Thunderbolt
- Amnesia / Ice Beam / Blizzard / Thunderbolt
- Recover / Rest
Why?: I got so lazy, and now that i've gotten used to the likes of Project 64 "Transfer Paks," I wanted to give this a try, so why don't we travel back when Mewtwo was once King of Gen 1.
- Psychic - This move is already standard and one of the broken resourses Mewtwo had potential of back then, as Psychic Types pretty much ruled the throne, so Mewtwo had had access to it as a STAB move, as they existed in Generation 1.
Substitute, Ice Beam, Blizzard, Thunderbolt - This is quite a long explanation between useful resourses, but Substitute, which became decently viable against your friends in Pokemon Stadium had to get a mention here, aside from the Amnesia combo you can recieve to bulk up Mewtwo. If you do not want to, however, Ice Beam and Blizzard make viable coverage moves, especially with Blizzards buffed Accuracy back then (90%). If that is not the case aswell, Thunderbolt is yet another viable coverage move, especially it had more "Powerpoints" than Ice Beam.
Amnesia, Ice Beam, Blizzard, Thunderbolt - The same 3 coverage moves can still be placed to have Mewtwo act as an "All-Out Special Attacker," but if you wish for more variety, Amnesia placed with Psychic and another coverage move works very well, as it is the most broken move back then, especially before the Special Split.
Recover, Rest - This one is an obvious choice in final selections, and Mewtwo is one of the more reliable Pokemon that can use Recover and simply dust off any HP it takes. If thats not the case, Rest does trade off a 2-Turn Sleep Status and, since its Generation 1, the ability to not attack in return for any Status Removals and HP Recovery.
There are limited Counters, but these are not limited to:
Thunder Wave: Electrode is viable with this move, and, being the fastest Pokemon in Gen 1, can easily bypass Mewtwo's Speed and Paralyze it before it even pulls off a Substitute to block it.
Hard Physical Hitters:: Pokemon that know such hard hitting moves like Explosion or Self Destruct can put a dent in Mewtwo's health as long as its left defenseless, but nonetheless Mewtwo can still pack a decent amount of Defense, live, and shrug it off with any recovery.
Special Walls: Chansey is one sort of Special Wall unless it goes up against Mewtwo for the worst in any other occasion.
There's nothing else to say of Mewtwo in Generation 1. Tied with Mew, is none other the best Pokemon in Generation 1, especially with access to broken resourses Like Amnesia and Recover and Powerhoused moves like Blizzard and Psychic.