One of the simplest and best mixed drinks ever devised, hence its lasting popularity.
To serve as a Highball use a smaller 10oz (295ml) Highball glass...
The best twist I’ve ever done on a gin & tonic is the addition of grapefruit zest. Just release the oils and discard, don’t leave it in the glass. Makes a huge difference. As long as you have a good tonic like Fentimans or Fever tree (prefer the Mediterranean for the Indian) any simple gin will do the drink. Both Gibson’s and Gordon’s make excellent Gin & tonics this way.