Wow, I wish I'd seen this thread before now. I have quite a whisky (and whiskey) collection.
Everyone who said you couldn't go wrong with Glenfiddich was right. The 12yo Ancient Reserve is the drink that took single-malt whiskys to the world and is still by far the market leader. Last I heard it accounted for two-thirds of all single-malt sales world-wide. If you wanted to impress her though, forking out for a 15 or 18yo bottle would do the trick. Tracking down a bottle of the 2010 limited edition "Snow Phoenix" would be even more impressive. Glenfiddich is my favourite scotch. I have 12, 15, 18, 21 and 30yo bottles.
Glenlivet is a nice scotch, as are Cardhu (the base scotch Johnnie Walker is blended from), Glenkinchie and Glenfarclas. Alternatively, you could have gone for an Irish whiskey such as Jameson 12 or 18yo, or a Bushmills 10yo single-malt. Bushmills Black Bush is also very good. The Irish whiskeys are made with out using peat when drying the malted barley, so they lack the smokey flavour ("peatyness") of most single-malt scotches. If your sister doesn't like the peaty flavour, Irish whiskys would be a good choice. Whiskey is actually Irish originally and was taken to Scotland from Ireland, so Irish whiskey is by no means an imitation of scotch. Bushmills' distillery is the oldest licensed whisky distillery in the world.
If she does like peatyness however, you could go for an Ardbeg, Lagavulin or Talisker.
Your post said "good whisky" and I see that everyone seems to have taken that as meaning "single-malt whisky", but there are plenty of good blended scotches out there too. I have Johnnie Walker Blue Label and Johnnie Walker Blue Label King George V Edition and they are both great. Same with the 18yo Chivas Regal.
Lastly, if you wanted to get her something in a stylish bottle, Jura: Superstition is an eye-catcher with a silver ankh on the front. The Dalmore looks good with its Stag's head on the bottle and comes in several ages depending on your budget. Haig's Dimple 12 and 15yo come in distinctive short, triangular, dimpled bottles. Johnnie Walker Swing comes in a bottle you can rock back and forth without it tipping over. Old St Andrews comes in a bottle shaped like a golf ball. The King George V that I mentioned earlier comes in a crystal decanter with a gold plated stopper, but it is a bit pricey. Which brings us all the way back to Glenfiddich. The tall, triangular bottles from the valley of the deer are the most distinctive and well known single-malt scotch bottles in the world... and the 12yo comes in a lovely shade of green.