Bitcoin (original, until August 2017)
The world's first cryptocurrency, about which much has been written. During its existence (until August 2017) it went from a few cents to several thousand dollars.
Bitcoin Cash
This is the most famous "alternative version" of bitcoin. It appeared in August 2017 at the same time as the updated BTC.
Bitcoin Gold
This is another recent bitcoin fork that appeared in October 2017. The main purpose of its creation was to simplify mining (availability to ordinary users without specialized equipment) and increase decentralization of the network.
BitcoinZ
Like BTG, this currency appeared in October 2017, so it is too early to judge its success. Unlike the above "bitcoins," BTCZ is not a fork of BTC, but of another currency - Zcash (ZEC).
BitcoinDark
Unlike BCH and BTG, this is a relatively old currency. It appeared in the times when BTC was faithful to Satoshi's principles in 2008, had no particular problems with speed and was criticized by few people.
Bitcoin (updated, from August 2017)
This currency is considered an official continuation of the original BTC, although technically it differs significantly from it. In August 2017, SegWit technology was added to the BTC algorithm, allowing some transactions to "bypass" the currency's blockchain.