I reckon the best knot for attaching fluorocarbon is the Albright knot. It’s a knot I’ve always had 100 per cent confidence in, and I’ve never lost a fish using it. I’d say that I use it for 90 per cent of my fishing.
It’s great for lure fishing but you’ll struggle to find a better knot to tie combi-links when carp or barbel fishing too. It can be a bit fiddly to tie at first but once you’ve practised it a few times you’ll be able to do it with your eyes closed!
There are plenty of instructional YouTube videos out there that show you how to tie it, too, but meantime here’s a diagram to be going on with.
Make a loop in the braid and run about 10ins of fluorocarbon through the loop. Hold the three ends of line/braid between your thumb and index finger. Wrap the fluorocarbon back over itself and both strands of the loop.
Make 10 tightly-wrapped turns. Feed the tag end of fluorocarbon back through the loop and exit the loop the same side as it entered
Hold both ends of the braid and slide the wraps to the end of the loop. Pull the fluorocarbon to tighten and clip tag end close to the knot.