If I had a choice it would probably be sex toys. Someone I know actually runs a fairly successful adult website specialising in sex toys and they showed me a catalog that they received from a whole saler once (and it was almost the thickness of a phone book), and the prices they pay for the stuff wholesale versus what they can be sold for retail is quite a good margin especially if you go for the generics and copy cats. The price doesn't even have to be astronomical either as the main reason the "private shops" charge so much is down to the fact they have to pay for licences as they sell R18 videos and magazines (it's also the reason that one of the main high street stores doesn't sell R18 videos, as they don't actually have a license).
It's not something I'd seriously consider setting up a business around though, due to the fact that you can get it online and even then a lot of the more established outlets have a loyal customer base, even if some also have "white label shops". These are basically affiliate shops that you can set up online in 5 minutes and while you may get a few orders, you'll never compete with the established ones.