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I would say Snorlax, just because he has good HP and defense, but he needs a little OOMF once he runs out of options. Mewtwo is a special attack powerhouse, so I don't see the need in teaching him, because his main attacks should be more powerful. (I'm pretty sure Self-Destruct is a Physical...
I miss the mechanics, MAINLY because when you got around, it was a whole lot simpler.
Surely there could be more areas, but adding too much route length kills it for me... Like the new Heart Gold/Soul Silver kind of ruins the whole route between Victory Road and New Bark Town...
Also, they...
You know, the list DOES explain what happened when I traded Missingno to my Silver version, and it turned into a Remoraid... which was weird, but it corrupted my file either way, and deleted the save file within my Silver Gameboy game.