So I lived in the Uk for 12 years and Gin is kinda a thing there... I also make my own gin (I have a small still). I haven't counted all of the gins I've had but its well into the three figures, most of them as a gin and tonic for the first time trying it so I get the gin profile and then see where it mixes best later.
First and foremost is getting good tonic water. For a long time here in the US it was Schweppes or Canada dry and that was it (maybe store brand). Then maybe 10 years ago you started to get more craft or higher quality. I'm a huge fan of Fevertree tonic. In the UK it was everywhere and it was relatively cheap (about $2 for a big bottle, not much more than schweppes). Here it is expensive- a 4-pack of 6.8 oz bottles is about $6 at Target- but its worth it. Standard gold bottle original is the best overall. The light is the best 'diet' tonic I've found and great for lighter gin flavors. Mediterranean is fun, we've just tried cucumber (my wife likes it), and the lemon tonic is great with sloe gin. If you don't want to get Fevertree, schweppes is my supermarket choice. Its enough quinine and cinchona without overpowering or turning you off. I'm not a fan of Q (too much cinchona/bitterness and too dry). I can deal with Canada dry but its a bit boring. My best friend prefers tonic syrup and soda water. I do not and struggle to get into it.
Ratios are personal preference. For me, a good 2-2.5 oz pour of gin and a 6.8 oz fevertree is about right. I might bump to 3 oz if I want a little more gin or down to 2 if its a navy strength gin. The more important thing is a big glass and lots of ice. In Spain you get served in a Copa, which is like a red wine glass in size and shape. Normally a dozen large ice cubes or more. You can scale up or down on the total drink, but make sure you have twice as much ice as liquid.
Garnishes need to match or complement the gin. Lime is good for a London dry gin and that's the bar standard for most all gins. Lemon is a good choice for london dry also especially if the gin has more of a coriander flavor. Lemon is also great with Malfi lemon gin. Grapefruit (either wedge or just an expressed peel) does well with slightly softer gins. Cucumber is the standard for Hendricks and pairs well with light profile gins also. Orange goes with sweeter or oriental /spiced style gins a lot of the time (Opihr is a good example) or any gins that use a lot of orange peel in their distillation. Herbs are a good choice too. Rosemary is an easy one since a stalk of rosemary stands up in a glass and is very fragrant on the nose without a ton of flavor added. Basil, thyme, and mint can all do the same thing. You can also get less traditional garnishes like olives (try that paired with rosemary and Gin Mare), mango, or watermelon cubes. The main thing is to not overpower the gin and tonic. I even had a cocktail shrimp as a garnish once (Lobstar gin which is distilled with lobster shells in the still).
Which Gin?
So many choices now. Lots of old standards and lots of upstarts. Probably the three market leaders (if I had to guess) for this purpose would be Bombay, Tanqueray, and Hendricks. All good in their category. Bombay is London dry style but has a lighter juniper character and a bit more coriander and lemon- definitely a crowd pleaser and one I buy by the handle. Great in G&T, makes a great martini, and still stands up enough in a negroni or other big cocktail. Tanqueray is London dry but much heavier on the juniper. People who don't like the 'pine' in gin won't like this. Tanqueray 10 is a more refined version. Hendricks saw an opening in the market for a floral gin and they added a ton of rosehip/rose petal/orris root to a fairly juniper gin. Lightened up with some cucumber its a very unique flavor profile and definitely a crowd pleaser.
Beyond the big three there are tons of choices. Malfi lemon is an Italian gin that is more like a lemon vodka than gin but with light tonic and a lemon peel it is sooo refreshing in the hot- almost like an alcoholic sprite. Gin mare is a Spanish gin that has heavy vegetal/green notes. Its great in a G&T with a green garnish like noted above, but also makes a great dirty martini. The Botanist is from Islay and is a very light, almost marine flavor gin that pairs well with light flavor profiles. Light fevertree, rosemary, and a grapefruit peel are the answer here. Sipsmith is based in Chiswick, west London, and we lived around the corner at one time. They have made a presence here in the US now. Very juniper forward, similar to Tanqueray but cleaner, maybe more creamy even. Monkey 47 is a complex gin with 47 botanicals. It's pricy and comes in a 500 ml bottle (so extra pricy per ounce), but has a unique flavor profile (so pair with regular tonic and a simple garnish). Opihr I mentioned above has a spiced profile with coriander, cardamom, bit of ginger even (not a ton of juniper). Great G&T, also very good in a martinez cocktail. Plymouth Navy strength (56% i think) is like a London dry on steroids. Good for a G&T (try it with Mediterranean fevertree), also does well in a negroni. Whitley Neil has some cool flavored gins- my FIL loves the quince flavor which makes a nice G&T. Hendricks does some seasonal editions (a friend just gave me Neptunia which I'll try tomorrow, mid summer solstice was great, Lunar was awesome). Aviation gin is an American gin, not too dissimilar to Botanist in the light profile category.
And my favorite you can't get here (edit- looks like total wine might carry it now! YAY!). Silent Pool gin is made about 15 minutes from our last UK house. Its lighter on the juniper and big on the orange peel, coriander, and lighter florals. We used to visit when they had two stills that were little bigger than a refrigerator set up in a converted barn. For a G&T, pair it with an orange wedge and fevertree gold.
For my own stuff, I have a london dry style I like and a citrus forward that is good. But my favorite of all of my own is an oriental spiced gin heavy on the allspice. Great in a G&T but crazy good in a martinez. I also did one that was very anise forward (I made a mistake in the recipe) that tasted like a cross between gin and ouzo/sambuca/black licorice. If you like that flavor you loved the gin. Not many love that flavor...