Italian · Bombay Sapphire
Bombay Sapphire uses vapour infusion: the spirit passes through a basket of botanicals as vapour rather than steeping in them. That gives a lighter, more floral gin with juniper that sits back rather than forward.

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The one thing that matters
In a negroni that restraint helps. Campari is loud and bitter, and a heavily juniper-led gin turns the drink into a fight. Sapphire lets the bitterness lead cleanly.
One that does not fight the Campari. Bombay Sapphire is light and floral, so the bitterness leads cleanly. A very juniper-forward London dry can make the drink feel crowded.
Traditionally yes, one to one to one. If you find it too bitter, take the Campari down to three quarters of an ounce before you change anything else.