anyway there have been many 'forks' of bitcoin. hundreds infact
XT was not a 'chain' fork because it was just a wallet software working on the network. but was a code fork because the code was similar
bitcoin cash was a 'chain' fork because it was forced to become an altcoin. due to the whole segwit bait and switch drama
before these there were many more, hundreds more.
the main first that retained the block history of bitcoin(prior to the fork) was called clams (a 'chain fork)
though there were also hundreds of others that just script-kiddy copied the code, but started again from block 1
so what answer is more like the truth? And if the question "Which crypto currency is the first fork of bitcoin ?"