Truth be told, there's no single answer, and be wary of anyone who tells you otherwise. You can pour Talisker 25 into a big cup of Sunny Delight if that's how you enjoy it best. However, there are a few things I've learned.
1. Some whiskeys are best enjoyed neat, so try them that way first
Prior to fussing with any preparation of the whiskey itself, take the first sip neat. It may be that if you're first starting out, even a whiskey at 40% ABV may taste pretty hot on the palate. If you find yourself making that whiskey face where your eyes bug out and your mouth puckers like a cartoon character, go ahead and add ice, water, or whatever else you'd like.
I have to admit that I've gone through a few bottles without appreciating what they truly have to offer simply because I'd been drowning them. Just off the top of my head, a delicate Japanese whisky like Hibiki 12 has really nice notes of plum, but they become indistinct as the spirit becomes diluted. The Balvenie Double Wood was never a favorite of mine because I thought for years it just tasted like an unchallenging bourbon. Sipping it neat brought out a lot more from the spirit and gave me a newfound respect for the malt.
2. Ice can be problematic
There are two reasons for this: first, anything that's too cold tends to have an anesthetizing effect on the palate. Here in Southern California, a quality bourbon poured over ice can sound pretty damned good on an evening where the temperature is still into the mid-nineties, but for most of the time ice will tend to make a drink chilled to the point where you do lose a good bit of flavor
The second and larger problem with ice is that it continues to melt. If you add a decent-sized cube to your drink intending to nurse it, you probably will hit that sweet spot where the harsh edge is knocked off the spirit. How long you hold onto that fleeting moment is a matter of debate, as your drink will invariably become weaker and more watery after that point.
When I first started out gaining an appreciation for whiskey, it typically involved Jameson on the rocks with enough time for it to mellow out and dilute. If there's ever an easy way to drink supermarket-grade whiskey, it's in just such a manner. I liked it because it mellowed out the already-sedate Jameson pretty well. However, as I actually began to acquire the taste of whiskey in general, my palate began to process the final sips from the rocks glass as being thin and watery rather than smooth and agreeable.
3. Water is best added carefully
You might be expecting to say that it's sacrilegious to add water to whiskey. Far from it! Many whiskeys benefit from the addition of water. Even just a few drops is enough to open the flavor up just a bit and release new tastes and aromas. With some drams, you almost get two different experiences for the price of one!
How much water to add is solely up to one's personal preference. Dalmore master distiller Richard Paterson thinks that tasting whiskey at 35% ABV is optimal for having the perfect sweet spot between drinkability and taste. I've read from other master tasters that 20% ABV is actually where the nose opens up most fully and allows you to experience a malt to maximum effect. For me? Anything at or under 43% is usually good straight out of the bottle, and even some cask-strengths can be surprisingly drinkable!
The other good news about water (unlike ice) is you can pour the whiskey into your glass, add water to taste, and know that your whiskey isn't going to become any different from that point forward. Best to add a little at a time, taste, and reassess. Whiskey guru Ralfy Mitchell gives a sage warning: once a whiskey is drowned, it's hard to bring it back with the addition of more whiskey from the bottle. Logically, this shouldn't seem to be the case, but my tastebuds and experience say he's onto something.
Rather than perilously dangle the open Aquafina bottle over your glencairn or old fashioned glass, there are a few better options. Either get little measuring beakers (I like these guys), use a 'lil baby teaspoon, or try transferring water from a drinking glass to your whiskey glass by creating a vacuum in your straw.
One final word of warning: be sure to use filtered or distilled water. There's no sense adulterating a scotch 18 years in the making with some supremely crappy tap water, which can often be metallic, sulfurous, or just plain dirty-tasting.