The best Mafia Wars attack animal (best attack stats): the Chain Viper.
Stats: 46 Attack, 33 Defense
Lootable: YES!
Where to get it: Bangkok, Episode 7, Silence A Shadow Kingdom Patrol mission
The best Mafia Wars defense animal (best defense stats): the Forest Scorpion
Stats: 25 Attack, 37 Defense
Lootable: YES!
Where to get it: Bangkok, Episode 4, Set Up An Opium Shipment mission
Also, there are two even better animals you can get in Mafia Wars, but these are not lootable, are expensive and quite difficult to get to. They are available in Bangkok (again), in the Faction Stores, meaning that in order to get them you’ll need to have an allied status with the Triads or Yakuza. However, it is definitely worth the trouble since their stats are:
Banded Krait: 51 Attack, 28 Defense
Xiamen Tiger: 26 Attack, 52 Defense
Did you get any of these top animals already?