How do I use superlatives the best, the worst?

Ciamar a chanas mi as fheàrr, as miosa?

If you want to say something is the best, then you need to use  Is e,  or  ’s e  – the assertive verb as you are asserting something. 

Is e seo am buntàta as motha
This is the biggest potato
Is e sin a’ chèic as lugha
That is the smallest cake
Is e siud na briosgaidean as miosa
Those are the worst biscuits over there