There are many knots that will do the job, some are stronger than others, some take longer to tie than others. For myself a leader connection knot needs to be easy to tie, strong and have a low finished profile so it casts well. This is not as important when fishing with light line classes as there diameters lend themselves to compact knots. The double uni is a fast reasonably strong knot thats dead easy to tie and works well under reasonable loads. You will find this knot will nearly always fail at the flourocaron side of the uni. Some knots like the Duck knows are great with monofiliment yet dont work with the denser, harder structure of flourocarbon, they simply slip and wont pull up. Flourocarbon is great stuff but it needs to be wet liberally and snugged up gently so as not to over heat and weaken the finished knot. The five tun surgons knot is fast,simple and a very realiable connection if tied properly. I use this knot for most of my soft plastic casting with leader weights up to 30 pounds or so. Above 30 pound leader I would use a slim beauty or an improved albright, once again strong connections that are reasonably easy to tie with a good low proflie. You can connect leaders of up 100lb easily with an  improved albright, once  you get above this sort of diameter we start to use wind on's as things get big and chunky. Grab one of Geoff Wilsons knot books for chrissie, there worth every cent. All of these knots are in there with excellent diagrams to teach you  how to tie them. Hope this helps

Cheers Captain