Definitely Hendriks for me, although I prefer it with Perrier & lime rather than tonic. Tonic seems to pretty much overpower the subtle flavors of Hendriks.
Bombay Sapphire will do in a pinch.
Darn, now I'm thirsty - off to mix a Hendriks martini.
I usually do Hendrick's as a martini, though if you are going to use tonic, try Fever Tree.