There is only one key that could have belonged only to the creator of the system (Satoshi), which is that associated with the Genesis Block. However, the coinbase reward associated with the block cannot be spent, as the Bitcoin Software does not allow that. So, while there is a public / private key pair for the Genesis Block, it is not certain that anyone has ever known the private key. Whilst early blocks are associated with Satoshi, they could theoretically have been mined by other early individuals right after launch. The Bitcoin community, however, does associate block 9 with Satoshi, because this block was the one involved in the first transaction from Satoshi to Hal Finney.