Lewis acids and bases

$BF_3$ has an incomplete octet on boron and accepts an electron pair, so it is a Lewis acid.
$PF_3$, CO and $F^-$ have lone pairs to donate.

A Lewis base is a species that can donate a lone pair of electrons.
$OH^-$ and $H_2O$ have lone pairs on oxygen, and $NH_3$ has a lone pair on nitrogen, so all three act as Lewis bases.
In $BF_3$, boron has only six electrons in its valence shell and an empty p orbital; it is electron deficient.
$BF_3$ therefore accepts an electron pair and behaves as a Lewis acid, not as a Lewis base.
Hence $BF_3$ is least likely to behave as a Lewis base.