Brazilian Rainbow Boa
Pros:
Gorgeous.
I want to get one eventually anyway.
Did I mention gorgeous?
Cons:
Expensive.
Will outgrow terrarium rapidly.
Rubber Boa
Pros:
Weird and awesome.
Docile and relatively easy to care for.
Possibly free. (And the experience of catching my own is not to be sneezed at.)
Will never outgrow terrarium.
Cons:
Not as pretty.
Poison Dart Frog
Pros:
Gorgeous.
Can have more than one in this space.
Can actually be handled, not poisonus in captivity.
Cons:
Frogs just aren't my thing.
Slightly expensive.
Spiny Flower "Number 9" Mantis
Pros:
Gorgeous.
Fairly cheap.
Cons:
Doesn't like handling.
Lives only one year.
Sort of technically illegal (It's like speeding 4 miles over or like pirating a few songs, nobody's ever going to go after you.)
Chinese mantis
Pros:
Large and pretty.
Easy to keep. (I have done so before, at the age of about 12.)
Amenable to handling.
Very cheap. Possibly free if I can find one.
Cons:
Lives only one year.
Not as flashy as other choices.