Tough question.
Usually Hobnobs (chocolate or not, although the best are dark chocolate) if dunking.
Custard creams and bourbons are pretty high on my list, although I'd probably choose a custard cream first.
I love shortbread and it's also really easy to make, meaning I can have it fresh from the oven.
Other than that, ginger biscuits. They can be dunked, but are equally delicious on their own.
tl;dr, Depends on my mood.