There’s always the impossible debate of which fly is or could possibly be the best fly for Atlantic salmon. If I had to pick one, I would probably go with Green Butt.
Atlantic salmon has a fairly good vision. After living in oceans (with green and blue shaded colors), salmon can see the shades of green and blue better when entering the river. Therefore many times the best early season patterns have some green and/or blue in them. Of course there are exceptions. For example big orange shrimp patterns which many time work also well in the early season.
Green Butt (GB) is a great and versatile pattern. You can pretty much tie any kind of salmon fly using the GB color scheme; hook flies, tubes, hitches, intrudes and even dry flies. They all work.
I tend to use fluorescent chartreuse to get the glowing effect. You can wrap some silver tinsel underneath the chartreuse silk to make it glow even after it gets wet. You can also play with the amount of green/chartreuse you put in the fly. Sometimes a tiny tail or butt does the trick, other times you can over-do the amount of chartreuse.