I can't think of an overall favorite, but here's a few where the bass line is so good I end up focusing on the bass line instead of the main melody, and I'll happily just jam out to the isolated bass stem:

Dean Town - Vulfpeck (ok, it's kind of cheating, but who doesn't wanna listen to Joe Dart play lead bass?)

Money - Pink Floyd (so simple, so good)

Platina - Maaya Sakamoto (Cardcaptor Sakura OP3) (incredibly lyrical yet groovy fretless bass)

Oh No, Oh Yes - Mariya Takeuchi (quintessential slow jam with the mood set by the synth bass; check out this bass cover for a breakdown (also just a great channel in general if you love bass--here's another great video))

Innocent Blue - DeviceHigh (admittedly a bit out of left field, but it's got a soft spot in my heart as the first song I ever transcribed and learned on bass)

For some reason my mind is going blank, but if I think of more I'll add them later.