probably dolphins or primates, just based on the things they can do and learn..

Apes, from the programs I've seen and articles I've read, they are amazingly intelligent. They can learn to spell and create art work. They also display many human like emotions.

The cat. No other animal can ignore humans so magnificently and still have the human tripping over themselves to wait on their every wish.

Elephants. They have fantastic memory, highly convoluted brains and a highly complex social life, have a low EQ, family orientation, and elephants have the lowest birth-to-adult brain ratio, indicating behavior is learned, not instinct (not the case with most other dolphins or animals). Elephants live in family groups (except for the males who join later) run by a matriach and her offspring, they use tools, they are altruistic, they grieve (they bury the dead, they pay homage, they cry), they self medicate (cetain leaves that induce labor), other females help aid a birth and help raise a calf (allomothering), they show complex emotions (love, joy, sorrow etc), they understand semantics/syntax in captivity (Koko was the one studied), they have even unlocked padlocks with twigs etc to escape from zoos. They are even known to bury sleeping humans from tribes in Africa with twigs and branches and will guard over them till they wake up. If you want references, I can give you the names of the books all this info is in. People tend to forget about elephants; they are so underrated. Research elephants and you will be amazed at what you find out.