Best toys for switches and best toys for M/s relationships?

I'm just looking for what everyone thinks is the best toy, accessory, costume, etc available for a switch couple, and the the best for a more traditional bdsm couple? Why do you think that? What are some must have toys for these kind of relationships? Does the sexual dynamic matter (male/male, male/female, female/female)? What about for poly couples? I'm adding a few pieces to my collection and willing to take suggestions on what's a must have vs. what's just crap.