Brawl CPUs learn a lot of your strategies in the case of a ditto, but Mario, Luigi and Bowser seem to have had their AIs made by pros. Mario and Luigi play around with their Nairs a lot and will combo you at the most random of times while Bowser will powersheild every single hit and allways follow up with a sheid grab.
Melee CPUs know how to combo you off the bat and on top of that, they know how to tech everything they do ;_;... oh yeah and Falcons love their Dairs. because of this, I can easily say that Captain Falcon, Fox and Ness are the hardest to beat. Falcon pulls his knee out of nowhere at really high %'s, Fox will **** you if he gets a grab off and Ness... Ness is just gay. The one flaw in Melee's CPUs is that they might taunt randomly, giving you an easy opening.