Rope
Versatile and classic. Silk ropes, cotton ropes, jute ropes (for shibari) all have different feels. Rope can be tied in beautiful patterns (like chest harnesses that accentuate the body) or simple loops. Always have safety scissors that can cut rope quickly if needed. Rope can leave marks (“rope burns” or indentations) which some like. There are many tutorials for cool shibari ties if aesthetic appeals.