Here's a comparison chart. The Human figures are Don & Judy, each with a Human, an Alien and a Stone texture. Genitalia is turned on. The robots and aliens are all completely unclothed. You vote which figures are nude by the TOS and which are not. I can almost guarantee you'll get two of them wrong.

Also, I didn't include the Darcassians because I don't own them. Are they even still available anywhere?