The bond strength $H-X$ decreases from $HF$ to $HI$. Thus $HF$ is the most stable and hence weakest acid while is least stable and most acidic. In aqueous solutions, $HF$ is only slightly ionised but $HCl, HBr$ and $HI$ are almost completely ionised. As basicity is the reverse of acidity. Therefore, $HF$ is the most basic of all $H-X$.