... certain/sure they were the best ...

would work; "certain to be ..." has a slightly different meaning, as does "sure to be..."

I agree with dude. The terms seem quite close in this context. There's a slight difference, but it's hard to describe.

See for example, http://www.merriam-webster.com/dictionary/certain #5, and http://www.merriam-webster.com/dictionary/sur... #4 and #5.

Of course, both certain and sure have other meanings which do not coincide between the two words.

Also, I want to point out that the original sentence is unclear. I don't know exactly what it means.

Does it mean that the the citizens of the island were sure they were the best?

The phrase "certain to be" or "sure to be" really describes a sense of expectation, rather than certainty. Example: Rolf was working late, and the kids were sure to be (or certain to be) in bed by the time he got home.