personally, i am in favour of saying that mysterial is indeed the best of the best and the toughest CPU trainer i've ever faced (few teams of my friends or even my own teams are better), and despite the knowledge i have of the identity of his teams, i still greatly struggle to beat him (have done a few times...but never as easily as he has done with me, hes beaten me an awful lot more than ive beaten him) i remember on stadium 2 battling your rival with his ho-oh, lugia and mewtwo (all of which pokemon mysterial has on his teams) thinking that this was probably the hardest battle ive been in against a CPU (probably wasnt but wasnt far off it because it is a 6-on-3 battle, but mysterial has surpassed it). i can spot a couple of questionable things with his team that i would do differently e.g. that awesome latios of his doesnt know psychic (not the end of the world, still tremendously powerful but you'd think it'd have it...but it doesnt). mysterial has 3 different teams when you face him in rank 4 and rank 8 (focusing on rank 8), unfortunately i've only ever faced 1 of those teams, the drizzle based team, and from what ive experienced i wouldnt be surprised to hear that this team is his powerhouse team. but what i ask is a simple matter of opinion, i personally believe he is, but what do you think? is mysterial the toughest trainer you've ever fought on any of the games?
his 3 rank 8 teams being: -
they look impressive on paper....try battling them.