I'll add more choices as I hear them -- but you have to explain why you think your animal is the ultimate.
Hippo:
1. They kill more people than all other animals combined!!!
2. They are so smart the get birds to keep them clean
3. They can run faster than Carl Lewis
4. They weigh more than a VW Bug
5. They eat and poop A LOT!!
6. They show their dominance by spraying their poo with their tails!
edit: I'll only include those with a reason as to WHY they are the best. -- you can't just throw out animals without giving a good reason
Hippo:
1. They kill more people than all other animals combined!!!
2. They are so smart the get birds to keep them clean
3. They can run faster than Carl Lewis
4. They weigh more than a VW Bug
5. They eat and poop A LOT!!
6. They show their dominance by spraying their poo with their tails!
edit: I'll only include those with a reason as to WHY they are the best. -- you can't just throw out animals without giving a good reason