+3 for the Perfection Loop. It's the standard knot for leader butt loops. It looks confusing, but it really is an easy knot once you get the steps, it tightens down easily.
The non-slip mono is an option as well, but is usually used as a terminal knot. It's a good one. Non-Slip Mono Knot | How to tie a Non-Slip Mono Knot | Fishing Knots