The most widely known is Pimms No.1. This is drunk in the summer time with lots of fruit, mint and loads of ice. Generally served in jugs.
Pimms No. 3 is also known as winter pimms. Served warm like a mulled wine.
Here is a list of the various pimms (so now you know)
Pimm's No. 1 Cup is based on gin. It is 25% alcohol by volume.
Pimm's No. 2 Cup was based on whisky. Currently phased out.
Pimm's No. 3 Cup is based on brandy. Phased out, but a version infused with spices and orange peel marketed as Pimm's Winter Cup is now seasonally available.
Pimm's No. 4 Cup was based on rum. Currently phased out.
Pimm's No. 5 Cup was based on rye. Currently phased out.
Pimm's No. 6 Cup is based on vodka. It is still produced, but in small quantities.