Writing on a book or an essay, anon?
My top two would be:
>Squids
They are insanely intelligent. Seriously, look it up; they have mammalian levels of intelligence.
>Monkeys/Apes
Again, very intelligent, and they are not far removed from humans, from an evolutionary viewpoint. We're not descendants of any contemporary apes, as far as I know, but maybe a certain potential for civilization is still in the contemporary ones.
>Crows
Pretty much the smartest birds that exist. They haven't conquered some habitats yet, but they could if they were smarter. What this means is that they actually have a reason to become more intelligent in the future, particularly if certain circumstances forced them to emigrate.
What I singled out was rats, as they breed too fast, are too resilient and hence don't need higher intelligence to overcome adaptionary problems.