getbestblockhash
Returns the hash of the best (tip) block in the most-work fully-validated chain.
Result:
"hex" (string) the block hash, hex-encoded
Examples:
> bitcoin-cli getbestblockhash
> curl --user myusername --data-binary '{"jsonrpc": "2.0", "id": "curltest", "method": "getbestblockhash", "params": []}' -H 'content-type: application/json' http://127.0.0.1:8332/