There's a lot of crappy tonic water out there.
Look at the label of any major commercial brand and you'll see high-fructose corn syrup -- not something the officers of the British Army were drinking in India when tonic water was first concocted as a malaria preventative (liberally spiked with gin and lime to make it more palatable). The nadir of supermarket tonic water comes out of the bottle labelled "Diet Tonic Water," with no sugar and with artificial sweeteners. Unbeknownst to me that was used to make my gin and tonic at a party once, and it took a Herculean effort to keep myself from spitting it out. (The remainder went into a potted plant. Such a waste of good gin.)
Fortunately we have some really nice small-label brands that are more interested in quality than in cheap ingredients -- Fever Tree, Q Tonic and Stirrings all make fine products. However, you can get much more of a depth of tonic flavor if you make your own.
Bartender Kevin Ludwig of Beaker & Flask in Portland makes his own tonic syrup from cinchona bark (nature's source of the profoundly bitter quinine), mixing it with plain soda for tonic water in his very flavorful G&Ts. You can order powdered cinchona bark from Tenzing Momo in Seattle (it's pretty cheap, and powerful; a little goes a long way). Here's the recipe for the tonic syrup, and the G&T is made thusly:
GIN AND HOUSEMADE TONIC
1/2 oz. homemade tonic syrup
1 1/2 oz. hearty, full-flavored gin
Fill a large rocks glass with ice. Build: pour in the tonic syrup and gin, top with soda water and a squeeze of lime and garnish with either a fresh lime wedge or lime peel.
I'd probably want to throw a dash or three of Peychaud's in there, too.
While you've got your cinchona bark, there's some more playing to be done.
The fortified wine we know as Lillet Blanc, as you probably know, started life as Kina Lillet. It was a quinquina, which are slightly bitter aperitif wines containing small amounts of quinine (hence the "Kina" in its original name). In the 1970s Lillet was reformulated, the quinine was removed, and it was rebranded "Lillet Blanc." It's still a lovely product, but that lack of quinine bite radically changed the flavor profiles of cocktails that were created with Kina Lillet in mind.
One of the more famous ones was the Vesper, or the so-called "James Bond Martini." He specifically calls for Kina Lillet when he's talking his bartender through it, as that was what was being made at the time. If you truly want to recreate a Bond-era Vesper, add a tiny pinch of cinchona to your mixing glass or shaker (we're talking 1/16th of a teaspoon or less). The results may astound out.