I use Schweppes diet as well, but was afraid to mention it.
Gin and Tonic: which are your brands, respectively? - Page 2
It is all relative, both to drink and location.
for G&T, i usually go Millers, but use westwinds tonic syrup mixed with soda water (for a stronger quinine that schweppes, but not as sweet)
drinking straight i always go melbourne gin co, on ice with a lime wedge.
martin depends. millers if going for a citrus twist. half tanq half tanq10 if going to a classic martini. and i go for westwinds sabre if going dirty
and tanq or melbourne gin co for negroni (depending on vermouth)
never bombay, far too spicy
and hendricks isn't my thing unless doing very specific cocktails