Man you make it hard to sacrafice anything in this question.
Hmmm if you said species it would be Emerald Tree Boas for me.
But you said 1 animal, then that would be my 1st Albino Boa - ZEUS - reason being the amount of time it took to finally get 1, and being my most desired and babied animal I had at the time in my collection. After trying 3 years to get Albinos this little guy brought me much joy and still does.
I got others but you did not make this easy to think out only 1, lol.