Which peers does a node broadcast a new block to?
All peers are treated equally when relaying chain-extending blocks (as James C. stated in a comment on another answer). The node will announce the block to all of its peers and since that block was just found, all of the peers should request the new block.
Many miners will also announce new blocks on Fast Internet Bitcoin Relay Engine (FIBRE) in addition. Any new block on FIBRE will be immediately broadcast to all participants in a compressed format based on compact blocks (BIP152).