British · Tanqueray
Four botanicals and nothing else: juniper, coriander, angelica root and liquorice. Charles Tanqueray settled that recipe in 1830 and it has not changed. It is the most juniper-forward gin most people will ever drink.

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The one thing that matters
This is the one that tastes like gin is supposed to taste. If a floral gin has ever left you wanting the pine back, this is the fix. Lime, not lemon, and plenty of it.
It is the benchmark. Four botanicals with juniper clearly in front, so it tastes unmistakably of gin even under a lot of tonic. If you want one bottle that makes a classic G and T, this is it.
A straightforward Indian tonic. Tanqueray is assertive enough that a flavoured or heavily botanical tonic just adds noise. Schweppes works; Fever-Tree Indian is the upgrade.
Lime with a juniper-forward gin like Tanqueray. Lemon suits softer, more citrus-led gins. With Hendrick's, use cucumber instead of either.