Gin has had a bit of a renaissance during the early 21ˢᵗ century. For many 20ᵗʰ century classic cocktails, the most traditional choices are going to be any British-produced, juniper-forward gins under the London dry gin classification and preferably at a high proof of 94.6, or thereabouts.
Some older recipes call for Old Tom gin which tends to be sweeter than London dry.
Other recipes call for Dutch jenever which tends to be sweeter than both.
Finally, there is Plymouth gin which is drier than London dry. The only gin in this style is from the brand, Plymouth.