Readers can correct me if I am wrong but I believe the first bitcoin hard fork is BCH. BCH is currently 3rd in market cap with a current worth ~0.185 BTC. The coin was created around July of 2017 and has slightly more coins mined than BTC. There are plenty of discussions as to why BCH was created all over the web but I will summarize it here in my own words.
As more users enter the BTC network, transactions were slower and transaction fees were rising. Furthermore with the demand of BTC rising came with rising price and hackers concentrated on finding exploits in the BTC blockchain. The major backers of BTC were outspoken about BTC's challenges and limitation as BTC network grew. Yet the backers were unable to settle on one path to resolving the problems. With some wanted to find ways to enhance the original BTC blockchain while others wanted to built a new BTC blockchain in lieu working around the existing due to inherent limitations. The end result was a hard fork of BCH. BCH was hardforked of bitcoin at block 478558 therefore any block at and before 478558 were un-minable BCH. Those BCH would go to current owners of those respective BTC.
Supporters of BCH believe it will replace BTC in the future while supporters of BTC believe the originality of BTC will maintain its lead in the crypto world.