Why is Fluoride anion less stable than iodide anion? Down the halogen group , electropositive character increases , then why is it that Fluoride ion is the least stable ? 
The counter arguement is that fluoride ion has four lone pairs and a small size which makes it least stable.Down the group , since atomic size increases the extra electron is stabilized.
In all such stability trends there are multiple counteracting factors , how to decide which one to give more weightage ?
Actually, the answer is a bit more nuanced. If we observe the pKb values (i.e. tendency to form HX from X-, where X is a B-L base) it is pretty clear that F- would form HF very quickly. 

The explanation is that I- would have a much larger area to balance out the negative charge compared to F-, and the anion would be more stable as a result.