Considering the big clubs are all north of the river, Chelsea definitely have the edge in South London support.
I was born in Battersea, grew up around Tooting, went to school in Sutton.
For me, it could've been Palace for a bit, but my best mate was Chelsea, his dad took us to a few games, so I made the right decision.
I live in East London now, which is a nightmare as blue sticks out like a sore thumb among the claret+blue and the black+white in the pubs round here.