The brand of gin is a personal preference, but do spring for a decent one; you’ll appreciate the difference. I definitely recommend using a good brand of tonic water, such as Fever Tree, Fentiman’s or Q. For a special treat, try making your own
homemade tonic water, which will give the drink a distinctly amber color. The amount of gin is a personal preference, and depends on the actual size of the glass. Some say 1 part gin to 2 parts tonic is right, but you can customize to your liking.