I'm partial to By the Way or Californication.
I like to listen to the "Dani Trilogy" as a set piece, with Californication, By the Way and Dani California (and my slight favorite of the 3 is Dani)
However, purely as a single track, I drift back to Soul to Squeeze - that is everything good about RHCP in one track.
I also like Snow and Under the Bridge. RHCP is a hella good band.