Best drink ever. Gin is pretty nasty on its own. Tonic water is pretty nasty on its own. Lime juice is pretty nasty on its own. Somehow mixing them in basically any ratio with any quality of ingredients produces a good drink. And it can be happily drunk whatever the weather, whether you're thirsty or just wanting a long sipping beverage. It's like miracle juice. It's something I can feel comfortable ordering in any bar and knowing I'll get something good. Haven't been proven wrong yet.
That said, for the money, Plymouth gin and Fever Tree Mediterranean Tonic Water with a squeeze of lime is the absolute tops. It will heal your soul.