report botnet C&C?

Assuming it is not a futile/wasted effort, where is the current best place/resource to report an active botnet C&C to?

Probably the abuse contact for the network in question, if contact info is available.

It may be worth reporting it to Shadow Server:

Team Cymru is always happy to hear about C&Cs, your best bet for
sending to us is to use

Tim Wilde

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Tim Wilde, Software Engineer, Team Cymru, Inc. | +1-847-378-3333 |

I don't know if there's a single best option, but there are several good
ones. In addition to Cymru I'd mention, which runs several
public botnet C&C trackers.