It's worth noting that if you're in the U.S. or Ireland it's "whiskey", and if you're in Canada or Scotland, it's "whisky".
The question is not exactly a straightforward one, because there are factors that would go in to this such as:
What price range are you talking about?
What time of year would you be drinking it?
What would you be using it for (a cocktail, on the rocks, drinking it neat)?
Are you talking a special occasional drink, or an every day whiskey?
Based on all of those it would be difficult for me to simply pick one favorite. Two I can say that are pretty much not worth bothering with for the most part are Canadian whisky and blended Scotch. There are some decent ones, but in general to me they really lack character. Canadian whisky is really controlled by a few major distillers, and doesn't have the breadth as some other styles/regions. Irish whiskey I certainly enjoy, and despite the fact that there are only three major distilleries left in Ireland (two in the republic and one in the north), there is a still a number of "brands" maintained by these three that differ in style and taste.
American whiskey (the term I use because Tennessee whiskey is not Bourbon) and Single Malt Scotch have the widest breadth of distillers and styles, and in my opinion produce the best specialty, aged and small batch whiskies. So I would probably term those two the "best", though generally I still prefer Irish whiskey to Scotch.
In what I would term the affordable range, i.e. under $40 for a 750 ml bottle, I would say it's hard to beat Woodford Reserve for a Bourbon. Fr. Chris mentioned Maker's Mark which I think is a good value too for a very good Bourbon, and I had a Maker's Mark julep this weekend. It's a little less sweet than some Bourbons because of the amount of wheat they use in the mashbill.
If you're really snobby, you can debate which type of still makes the best whiskey.
None of this however deals with the question of what is the best Tequila.
