This is a difficult question - what is an accessory? In the dealers shop, almost everything is an accessory, but the products can be divided between 'needs' and 'wants'. I need a leisure battery and a water carrier, so these would tend to be my 'top accessories', but hardly something interesting to be excited about.
So I guess the question is really about those 'wants' or inventions. My own favourite is my adaption of the humble decorators plumb bob for levelling - its simple, its truly useful, its accurate and it never breaks. But the follow up to this is why don't manufacturers build in such a device, when their customers have to do level their products on most trips?