In my opinion, Sapphire is pretty inferior for gin and tonics, but
is okay for a martini occasionally (although nowhere near the best).
Beefeaters makes a somewhat better traditional martini, a vastly better
G&T, and costs considerably less. A step up from Beefeaters is Boodles.

Then there is the category of "martini gins" designed to be used
without vermouth. A very good straight-ahead example is Junipero.
Some others are Hendricks, 209, Tanqueray 10, and the newly-introduced
Tanqueray Rangpur. All of these have too many botanical flavors
to be a really good choice for a G&T, but may have what you want
in a martini.

Steve