Koffing might not be the strongest but it has the best assurance of a side affect other than those auto sleep or poison Pokemon attacks.
 
HP:  50
Weakness:  Psychic
Resistance:  None
Retreat cost:  1 Colorless energy
Attack:  1 attack-2 Grass Energy
 
Foul Gas:  Flip a coin if heads the Pokemon is now poisoned; if tails the Pokemon is now confused.
 
Since both sides of the coin do a certain side-affect, there is almost no way you can't cause significant damage to that Pokemon (at least you do damage with poison if not with confusion).
 
E-mail me at the_master1080@hotmail.com.
 
Master1080