According to the following:
https://www.biscuiteers.com/us/blog/backstage-at-biscuiteers/the-difference-between-cookies-and-biscuitsBiscuits are "a light, sweet flour-based food product. Unlike a chewier cookie, biscuits are made with a dough, they can be cut into any shape and their firmer, snappier and flatter constitution provides the perfect blank canvas for icing and decorating", according to the UK definition. In the US, their equivalent would be known as either a cookie or cracker.
Cookies on the other hand are "a larger, softer and chewier creation. Baked using a softer dough, cookies in the British sense are often loaded with chunky or chopped ingredients like chocolate chips, dried fruits, caramel"
"American biscuit is a crumbly leavened quick bread similar to what we call a scone in the UK".
So I would say Oreo is more of a biscuit under the UK definition, and in the US it's a cookie. Under US definition, UK biscuits are US cookies, but not all US cookies are UK biscuits.
My contenders for best biscuits:
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Amaretti - particularly well paired with plain coffee (no sugar)
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Rich Tea - good with tea (no sugar)
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Chewy home made cookies filled with either chocolate (milk or white) bits or caramel chunks - if cookies are included in this category.
All the above contain too much sugar though.