I like any good London Dry for a G&T (e.g. Tanqueray, Beefeater) as opposed to something newfangled like Hendrick's. Though I have a shameful fondness for Tanqueray Rangpur & tonic, it's basically like drinking lime candy...
Boodle's and Plymouth are both of my go-to gins for pretty much everything, but ESPECIALLY G&Ts. They're deliciously dry with just enough juniper to flavor without overpowering.
Haven't had plymouth, but on the advice of another thread I picked up a bottle of boodles, and I must say it's quite tasty in a G+T. Tanqueray is my usual gin of choice, but Boodles does right by me, and it's a teensy bit cheaper in NYC.
Frankly New Amsterdam, it's substantially cheaper and a taste comparison between it, Tanqueray, and Plymouth produced this as a winner. Plymouth was just a bit too flowery for it, but made a really awesome martini!
Plymouth, Tanqueray is my favourite gin, but it doesn't cut through tonic enough for me. Better served over ice or straight up and ice cold. Plymouth (especially the naval strength) packs more punch to match a good tonic.
More important to me is what tonic to use, I like fever tree sugar free tonic. Dry, and very well balanced. Schweppes is a definite no no if I get a choice.
Gordon or seagrams. Why dilute good plymouth or beefeaters with tonic and lime? The things that make them special in a martini dissapear in a g and t. Plus where I live, we drink gintonics (as they are pronounced) almost constantly, and good gins price adds up