It's rare that I've had a gin I don't like, so I just tend to buy what I see on offer and always have a few in.
In the dim and distant past when I was able to do crazy things like go to the Canaries on holiday, I'd pick up a bottle in duty free on the way back. Best bargain was a litre of Tanquery 10 for under £25.
Waitrose is good for offers on 'fancy' gin, especially if you combine with the money off vouchers they send with their My Waitrose card.
Recently, I've bought a few bottles from The Drop Store online, which has some really good offers. Best bargains have been 2 bottles of Liverpool gin for £45, which is almost half price. Also a Welsh orange (Aber Falls?) marmalade one, which is really cheap.
I also like my gin to taste like gin, and tend to stick to normal ones, or spiced ones like Ophir, or citrus ones, I especially like the Seville Orange Liverpool one. I don't like pink strawberry gins, most gin liquers, or anything flavoured with elderflower, so tend to avoid those.
For tonic, my preference is Waitrose in cans, and I agree with a PP that most other tonics aren't fizzy enough.