The general rule is that strong acids produce weak conjugate bases and weak acids produce stronger conjugate bases. For the equation for a general acid below:
HA --> H+ + A-
HA is an acid, H+ is the proton it produces, and A- is the conjugate base. For a strong acid like HCl, the conjugate base Cl- is a very weak base. The halogen based acids (HF, HCl, HBr, and HI) are all strong acids so their conjugate bases are weak. NH4+ is a weaker acid, therefore its conjugate base NH3 is stronger. I can tell you from a practical standpoint that chemists often use ammonia (NH3) as a mild base but never use fluoride (F-) as a base. So the answer is that NH3 is a stronger base than F-.