Best graphics card up to 300 euro
Have a ryzen 7 1700
16gb ddr4
Want to play fortnite and stream without lags
The Vega 56 is currently quite popular and will certainly be enough for Fortnite.
GTX 1660 ti, or Vega 56, if you do not care about the consumption.
Here is a comparison of the GTX 1660 Ti and Vega 56