I have fished all over the world and the one lure that I can depend on to catch fish is the beetle spin. In a pinch it is always my go to lure. They can be had in multitude of sizes and are compact and easily fit into the smallest kit.

PS: I don't like soft plastics in a kit that I am not going to use much, over time and with a little heat the can get sticky and make a mess.
Someone also asked for some fly patterns that work, here goes my favorite list. Nymphs: Telico, Hares ear, Pheasant tail, Woolly booger. Dry flys: Adams, Ginger quill, Mosquito, Irresistable, Stimulater, Thunderhead. I usually fish these in a size 10 or 12 and have caught trout and panfish on all of these patterns.