Not sure which cryptocurrency has the fastest transaction speed but some blockchains come close. For example: 

1. Nano - The transaction speed is almost instant. 
2. Stellar - Takes around 2 seconds. By the time you check the transaction id on Stellar blockchain, it should already have sent and approved. 
3. Ripple - The transaction speed is around 2 seconds 

There are many other blockchains that I can add, but I hope you get the idea. 

That being said, it's also important to consider the number of transactions any blockchain is capable of handling. For example, Ethereum had a pretty good transaction speed back in 2016. As the network load slowly increased, the transaction speed kept on becoming slower.