Uni to uni or albright knot
If you use a uni to uni make sure you double up the braid to prevent the braid cutting through your leader . The uni to uni knot will not slip but as i said before it tends to cut into itself where as the albright knot will slip before it breaks (roughly 80% of the main lines breaking strain so it shouldn't be too much of a problem as you tend to set your drag at max 35-40% but normally 20-25%). You'll find that most people will use either of these knots.