Once your engine deviates from stock there are a lot of choices you can look at to optimize the performance. With a "stock" engine, if you are running resistor type spark plug wires, you will be happy with NGK BP6ES. If you are using solid core spark plug wires, use the NGK BPR6ES that Cal844 suggested. They are basically the same plug but the one with the "R" in the part number has added resistance. Others will certainly recommend other plugs that they are happy with.
Your questions suggest that this may be a bit new to you. While you are at the store buying the socket and extension, also buy a set of plug gapping tools/gauges. The plugs are not necessarily gapped correctly for the Mini when you take them out of the box.
Gapping tool/gauge

Video for Gapping Method and Use of Tool