Porygon - Z, is simply the strongest in my opinion.
Snorlax, who can cause severe damage (both to opponent and your fridge)..
Slaking, who, when not scratching his ass, can hurt you from both sides of the spectrum...
Tauros, cuz a rampaging bull is just plain badass...
Togekiss...damn Serene Grace abuser.
Smeargle is fun too (who doesn't want a paint brush for a tail?).
& of course Blissey, but I hate her.