It really depends on if you actually need the "fastest" and what you do with your router. If you have only say a 50m internet connection and you do not have a file server in your house likely any fairly modern nic will work fine. You are being limited by the internet connection well before your wireless will have a significant impact on the speed.
Still if you actually want the fastest you can look at something like
https://www.asus.com/Networking/PCEAC68/
There are other companies that sell similar cards from say tplink or netgear that cost a little less but are similar. The key feature is the ability to extend the antennas away from the back of the computer. This will in many cases greatly increase your signal levels and...